John Knox was born in a street named "Giffordgate" in the Scottish town of Haddington in East Lothian, some time in 1514. His father, William Knox, was a merchant or craftsman, but little is known about his mother, who was probably one of the Sinclairs of Northrig. He had a brother, William, who later became a successful merchant too. Knox's parents died in unknown circumstances when the boys were still young, and they were taken in by a wealthy family who enabled Knox to leave home at the age of 15 and go and study in St Andrews in Fife. Although Knox had already been exposed to the growing wave of Protestantism, he became a priest when he left university, as had been decided for him. However, the seed of Knox's conversion had already been sown, and he was further impressed by the travelling preacher, George Wishart whom he befriended. In 1546, when Wishart was tried for heresy and burnt at the stake by Cardinal David Beaton, Knox went into hiding. Cardinal Beaton was murdered and the Castle of St Andrews besieged by the murderers, and Knox was encouraged to take on the role of preacher too. His fluency, sarcasm and directness earned him popularity, and he set about denouncing the corruption of the Roman Catholic Church, using the Scriptures as his source. By July 1547, the French fleet summoned by the Scottish Queen Regent, Mary of Guise (Mary, Queen of Scots' mother) came to the rescue and retook possession of St Andrews Castle. Beaton's murderers were captured and sent away as galley slaves, including John Knox. Knox used his time in the French galleys to strengthen his new faith until his eventual release. Back in London, the Privy Council saw in him an opportunity to spread Protestantism and sent him to preach in Berwick-upon-Tweed. There, in the parish church of St Mary, he attracted crowds of recent converts, curious Catholics and Scots, who crossed the border illegally to listen to him.
One of the issues dear to Knox concerned the doctrine of transubstantiation. According to Catholics, the bread and wine consecrated during the Mass physically changed into the body and blood of Christ. The reformers did not agree with that, nor with the use of incense, bells, solemn music and colourful vestments during Mass. Latin should be done away with and the word Mass itself should be replaced by Communion or Lord's Supper. Whether people should kneel or sit while receiving the bread and wine later proved to be an irreconcilable bone of contention between Knox and other fellow Protestants.
Knox's popularity took him to Newcastle in 1551 and to the court of Edward VI after the death of the Duke of Somerset. Refusing the bishopric of Rochester, Knox returned to Newcastle where his sermons became increasingly politicised. He made himself a few powerful enemies by accusing statesmen of undermining the Protestant Church.

Knox's misogyny is legendary. Behind this blatant hatred however, lay a more complex persona. His relationship with Elizabeth Bowes has long raised some eyebrows. About ten years older than Knox, Elizabeth was married to Richard Bowes, Captain of Norham Castle, when she was only 16 and bore him numerous children. Although a devout Catholic, she became increasingly troubled and converted to the new faith. She was first introduced to Knox when she went to listen to him in Berwick in 1549. Elizabeth had particular difficulty with the Protestant doctrine of predestination, and sent Knox long and mournful letters requesting his advice. The relationship probably remained that of preacher and parishioner, although the tone of Knox's letters throws a definite doubt on that. Whatever the case, it seems that Richard Bowes was not overly perturbed by his wife's friendship with Knox. Elizabeth may have played more the role of mother than mistress to Knox, and she decided that he should find himself a wife. Her choice was her own fifth daughter, Marjorie, an educated girl of 16 or 17. There is no doubt that Knox, an impoverished man of 35, saw this as a golden opportunity, which is probably why her father was reluctant to give his consent. Knox and Marjorie were betrothed but Richard Bowes refused to complete the marriage contract and hand over the dowry. By that time, King Edward was seriously ill and the prospect of his half-sister, the Catholic Mary Tudor, succeeding him must have made Richard Bowes even more unwilling to have a renegade priest for son-in-law. Knox commuted between the southern counties and Berwick, keeping an active correspondence with Elizabeth. On the accession of Mary Tudor, Knox thought it prudent to flee to the safety of France, not without guilt for having forsaken his congregation. The politically tainted sermons now began to take the shape of open incitation to rebellion in Knox's mind. Still unsure of himself, he travelled to Geneva to consult the leading Protestant authority, John Calvin.

Although Calvin echoed Knox's opinion of women, believing that a female monarch was a punishment from God for the sins of mankind, he fell short of recommending disobedience. Disappointed, Knox continued his tour of the Swiss theologians until he had gathered enough justification for his new line of attack. Back in the French town of Dieppe, he started publishing various anonymous tracts advocating resistance to Mary I, which were smuggled across into England. In late July 1554, Knox returned to Geneva with the intention of studying, but was quickly prompted by Calvin to go to Frankfurt in Germany, where a group of English Protestant exiles had chosen him to be their pastor. He took his duties in the autumn of 1554 at the Church of the White Ladies but only to find himself embroiled in academic disputes over which form the service should take. A huge rift developed between the Protestants who supported the Common Book of Prayer and those who did not. Eventually, Knox gave up and returned to the peace of Geneva, but only a few months later he was called back to Scotland by the Scottish Protestants who needed a leader to secure the Reformation. Knox was not keen to go but Elizabeth Bowes managed to change his mind and he arrived in Edinburgh in September 1555. His arrival provoked a great deal of interest, and he divided his time between preaching and meeting up with influential public figures throughout Scotland. Besides attacking the Catholic practice of fasting during Lent, he also proclaimed that tithes should no longer be paid to the Church, as the proceeds were not spent on the poor. Mary of Guise, fearing a riot, sheltered Knox from prosecution. Knox saw this as an opportunity to show the Queen Regent the error of her Catholic ways, and he sent her a letter to this effect. Mary of Guise was not impressed by this agitator who dared lecture her as well and said to James Beaton, Archbishop of Glasgow: "Please you, my Lord, to read a pasquil (satire)". Knox never forgave her for that remark.

Soon, Knox was once more called to Geneva to be the leader of the congregation of exiles there. He readily accepted, sending Elizabeth and Marjorie (whom he had recently married in Edinburgh) ahead. He joined them in Dieppe accompanied by a servant and a student, and reached Geneva in September 1556. He was reunited with some of his colleagues from Frankfurt and could once more resume his preaching at leisure. Elizabeth became his housekeeper and Marjorie his secretary. Their first son, Nathaniel, was born.
Another friend of Knox's then arrived from London, Anna Locke. Anna was some fifteen years younger than Knox, and like Elizabeth, thirsted for Knox's spiritual guidance in the form of long and frequent letters. Again, the language and tone of the letters raise suspicions, but Anna's husband was quite happy to let her travel to the safer Geneva. Knox continued to keep in contact with her from wherever he was throughout the years.

In May 1557, Knox was once more dragged away from his congregation by the Protestant nobility, but only to find that he had made a wasted journey to Dieppe. Furious, he stayed on for a while, expressing his concerns about the ambiguous attitude of the Lords who, motivated by self-interest, had accepted the marriage of the young Mary Stuart to the French Dauphin, François, and the influence of the Duke of Châtelherault. Back in Geneva three months later, he began writing his most popular works "History of the Scottish Reformation" and the infamous "First Blast of the Trumpet against the Monstrous Regiment of Women". In the latter, he attempts to demonstrate the inferiority of women and rambles on about his favourite subject of women and the state, openly attacking the rule of Mary I of England. Knox conveniently avoided mentioning this work to Calvin, in the knowledge that he would never approve such revolutionary ideas. Mary Tudor's response to the First Blast was to ban imports of seditious and heretical books into England, while Protestants at home and abroad were shocked at the tone Knox was taking. Calvin dissociated himself completely from it and banned its sale in Geneva. Knox also found it necessary to publish various other tracts against Mary of Guise, relishing the loss of her husband and two baby sons, which he proclaimed, was God's punishment for her sins. Knox had become an extremist, inciting people to violence against their ruler.

When Elizabeth I succeeded Mary in 1558, she was also infuriated by Knox's insubordination and views against female rulers as a whole. She refused to grant Knox a safe conduct through her realm when he was recalled to Scotland at the end of the year. Knox complained in vain and had to undertake the journey by sea instead, landing at Leith in May 1559. Marjorie, who had given birth to their second son Eleazer, stayed behind in Switzerland. Knox found Scotland in a state of turmoil. Mary of Guise no longer believed that the Protestants' demands were motivated by genuine religious reasons but rather by a political agenda. She intended to have the dissident preachers outlawed and banished at Stirling, but Knox and the Lords marched on Perth. There, on 11 May 1559 from the pulpit of St John's Kirk, Knox preached a violent sermon against idolatry, which later led to the widespread destruction of the contents of the same church and local friaries. Knox was jubilant but later dissociated himself from the damage caused by attributing it to the "rascal multitude". Mary of Guise was forced into peace negotiations with the Scottish nobility but Knox still continued to vociferate against her and incite the mob to violence. So much so that Mary of Guise was compelled to seek French help while the Lords opened up negotiations with England, replacing the tactless Knox with Maitland of Lethington as their emissary. Elizabeth remained wary and unwilling to be seen to be supporting rebels against their ruler but eventually, English forces arrived in Scotland and the siege of Leith followed. Reunited with Elizabeth Bowes and Marjorie, Knox continued to spit his venom from St Andrews. Mary of Guise had retired to the safety of Edinburgh Castle and was seriously ill with dropsy. This did not stop Knox who, alleging that she had gloated at the sight of English corpses hung on the walls of Leith from the windows of Edinburgh Castle (which anyone who has ever been to Edinburgh will know is impossible), said: "within a few days thereafter, began her belly and loathsome legs to swell, and so continued till God did execute his judgment upon her." On 7 June 1560 Mary of Guise succumbed to her illness and the Treaty of Edinburgh was signed. By 10 July 1560, Parliament passed a series of Acts which would make Scotland an officially Protestant country. The First Book of Discipline, on which Knox and other ministers had been working since the previous spring, contained detailed proposals concerning the policy and discipline of the Church of Scotland. These wide-ranging proposals were passed on 27 January 1561, but it would be a long time before these were fully implemented. Knox turned down the post of superintendent and was given comfortable lodgings in Trunk Close off the High Street, together with the highest salary payable to a minister. In November 1560 Marjorie died, leaving Knox depressed and struggling to cope with his two sons. Mary of Guise might be dead and Scotland a reformed country but the French danger was still present in his mind. The French Dauphin had refused to ratify the Treaty of Edinburgh and there were rumours that the French were planning to send more forces to Scotland. Knox was relieved when he heard of the Dauphin's death in December 1560, but saw the return of the Catholic Mary, Queen of Scots with great foreboding. He was deeply suspicious of her from the very beginning and always refused to treat her as a fellow human being. His negative attitude towards her would grow into a most obsessive and unchristian hatred. Mary, who had just lost both her mother and husband at short interval, had no sympathy for this evil man who had made her mother's life hell, and put his despicable views to paper in the First Blast. She wanted him banished from Scotland, and her half-brother Lord James, knew that there was no way she would set foot in Scotland unless he obtained some sort of written apology from Knox. Knox, who did not know the meaning of humility and forgiveness, refused to apologise but pretended that the First Blast had been written with Mary I in mind and no one else. This of course did not prevent him from saying about Mary when she first returned: "The very face of heaven the time of her arrival did manifestly speak what comfort was brought unto this country with her, to wit, sorrow, dolour, darkness and all impiety." Although Mary always showed great tolerance towards the new religion and went out of her way to compromise with the Protestants, Knox was incapable of showing this level of understanding and conciliation. Within days of her return, he was denouncing the Mass which she was attending in private in the Chapel of Holyroodhouse, and spewed his hatred from the pulpit of St Giles at every opportunity. Nothing which Mary said or did obtained his approval, but few of the Lords agreed with his outright condemnation. Mary subjected herself to five interviews with Knox, without positive outcome. For all his hatred of women, Knox found it impossible to cope on his own and, on 25 March 1564, he caused scandal by marrying Margaret Stewart, the 16-year old daughter of his friend Lord Ochiltree, and niece of the Duke of Châtelherault. Again, Knox had shown where his preferences lay: youth and social standing. As for his two sons, they were sent to be educated in England with their Bowes relatives. During the seven years of Mary's rule, Knox managed to fall out with just about everybody, including Lord James Stuart, Earl of Moray and half-brother of the Queen. The General Assembly even convened a special meeting in an attempt to tone down Knox's attacks against Mary, but it broke down in disarray with nothing achieved. He disliked Darnley and hated Riccio, and openly approved of the murder of the latter, cautiously leaving for Ayrshire to avoid prosecution. After the birth of Mary's son in 1566, Knox went to England to visit his sons, and back in Scotland after Darnley's murder, lost no time in demanding that Mary be put to death for her sins. Even after Mary's forced abdication, Knox continued to preach against her five times a week, and his paranoia persisted even after her flight to England. "We look daily for the arrival of the Duke and his Frenchmen, sent to restore Satan to his kingdom in the person of his dearest lieutenant.", he wrote. Knox was disappointed by Elizabeth's lax handling of Mary and constantly feared a plot to assassinate the Regent Moray or Mary's son, the little King James VI. More than ever, he wanted Mary dead: "If ye strike not at the roots, the branches that appear to be broken will bud again (and that more quickly than any man can believe) with greater force than we would wish." When Moray was eventually murdered, Knox reduced the congregation to tears by praising his virtues at his funeral.

During all these events, Knox had been leading a content and comfortable life with his young bride, who had given him two daughters, Martha in 1565 and Margaret in 1567. Like Marjorie, she was an educated and devoted wife who helped him with his paperwork and entertained his many guests. She bore him a third daughter Elizabeth in 1570. In the autumn of 1570, Knox suffered a stroke but continued to demand Mary's death, rendering him unpopular with the more moderate Protestants. A former friend of his, Kirkcaldy of Grange, had switched over to the Queen's party, and on 30 April 1571, issued a proclamation ordering all supporters of the King's party to leave Edinburgh within six hours. Knox refused at first but was finally persuaded to go in May 1571, and left with his secretary Richard Bannatyne and his family. In St Andrews, he went on ranting and raving from the pulpit despite his old age and ill health, attracting both admiration and dislike from the academic community. On 31 July 1572, the King and the Queen's parties signed a truce and Knox returned to Edinburgh, where he resumed his preaching and lecturing from St Giles Cathedral. He finally died of pneumonia on 24 November 1572 at his home in Trunk Close. He was buried in the former cemetery behind St Giles.