He was a pupil of Erasmus, who called him inter nobiles doctissimus. His friends included Colet, More and Grocyn. He held a command in the force sent to suppress Perkin Warbeck's rebellion in 1497. In 1513 he was appointed governor of Tournai, and his letters to Wolsey and Henry VIII describing his vigorous government of the town are preserved in the British Library.
He was present with Henry VIII at the Field of the Cloth of Gold in 1520, and at the meeting with Charles V in 1522. He had been master of the mint since 1509, and chamberlain to Catherine of Aragon since 1512. It fell to him in this office to announce to the queen Henry's intention to divorce her; he also signed the letter to the pope conveying the king's threat to repudiate the papal supremacy unless the divorce were granted. Mountjoy, who was one of the wealthiest English nobles of his time, died in 1534. His son Charles, 5th Baron Mountjoy (1516-1544), was also a patron of learning.