Barnes was born in King's Lynn, Norfolk in 1495, and was educated at Cambridge, where he was a member of the Austin Friars. Sometime after 1514 he was sent to study in Leuven. Barnes returned to Cambridge in the early 1520s, where he graduated Doctor of Divinity in 1523, and, soon after, was made Prior of his Cambridge convent. John Foxe says that Barnes was one of the Cambridge men who gathered at the White Horse Tavern for Bible-reading and theological discussion in the early 1530s. At the encouragement of Thomas Bilney, Barnes preached at the Christmas Midnight Mass in 1525 at St Edward's Church in Cambridge. Barnes' sermon, although against clerical pomp and ecclesiastical abuses, was neither particularly unorthodox nor surprising. However, seeing a churchwarden whose civil suit resulted in the imprisonment of a local man, Barnes departed from his prepared text to denounce lawsuits by one Christian against another—and this in a church traditionally associated with the lawyers' college. Coming at a time when Cardinal Wolsey was attempting to stop the infiltration from the continent of copies of Luther's works, Barnes' remarks appeared suspect. As a result, in 1526 he was brought before the vice-chancellor for preaching a heterodox sermon, and was subsequently examined by Wolsey and four other bishops. He was condemned to abjure or be burnt; and preferring the former alternative, was committed to the Fleet prison and afterwards to the Austin Friars in London. Although confined to the house, he was allowed visitors. It was subsequently discovered that while there, Barnes became a known distributor of contraband copies of the Bible in the vernacular. He escaped to Antwerp in 1528, and also visited Wittenberg, where he made Martin Luther's acquaintance. He also came across Stephen Vaughan, an agent of Thomas Cromwell and an advanced reformer, who recommended him to Cromwell: "Look well," he wrote, "upon Dr Barnes' book. It is such a piece of work as I have not yet seen any like it. I think he shall seal it with his blood". In 1531 Barnes returned to England, becoming one of the chief intermediaries between the English government and LutheranGermany. In 1535 he was sent to Germany in the hope of inducing Lutheran divines to approve of Henry's divorce from Catherine of Aragon, and four years later he was employed in negotiations connected with Anne of Cleves's marriage. The policy was Cromwell's, but Henry VIII had already in 1538 refused to adopt Lutheran theology, and the statute of Six Articles, followed by the king's unsuccessful marriage to Anne of Cleves, brought the agents of that policy to ruin. An attack upon Bishop Gardiner by Barnes in a sermon at St Paul's Cross was the signal for a bitter struggle between the Protestant and reactionary parties in Henry's council, which raged during the spring of 1540. Barnes was forced to apologise and recant; and Gardiner delivered a series of sermons at St Paul's Cross to counteract Barnes' invective. But a month or so later Cromwell was made earl of Essex, Gardiner's friend, Bishop Sampson, was sent to the Tower, and Barnes reverted to Lutheranism. It was a delusive victory. In July, Cromwell was attainted, Anne of Cleves was divorced and Barnes was burnt. Barnes was one of six executed on the same day: two, William Jerome and Thomas Gerrard, were, like himself, burnt for heresy under the Six Articles; three, Thomas Abel, Richard Fetherstone and Edward Powell, were hanged for treason in denying the royal supremacy. Both Lutherans and Catholics on the continent were shocked. Luther published Barnes' confession with a preface of his own as Bekenntnis des Glaubens. Some historians have concluded that Barnes was crucial in having the English Protestants and Catholics alike understand the Reformation around them.
