English lord whose quarrel with Henry of Bolingbroke, Duke of Hereford (later King Henry IV, reigned 1399–1413), was a critical episode in the events leading to the overthrow of King Richard II (reigned 1377–99) by Bolingbroke. The quarrel dominates the first act of William Shakespeare's play Richard II.

The son of John, 4th Lord Mowbray, Thomas was made Earl of Nottingham in 1383. Several years later he joined the group of powerful nobles—known as the lords appellant—who from 1387 to 1389 forced Richard II to submit to their authority. Nevertheless, after Richard regained power, he employed Mowbray on military and diplomatic missions. In 1397 Richard arrested three leading appellants, including Thomas of Woodstock, Duke of Gloucester. Committed to Mowbray's charge, Gloucester was mysteriously murdered, possibly on orders from Richard.

Although Mowbray was then created Duke of Norfolk in 1397, he feared that the king would have him arrested for his earlier disloyalty. He confided these fears to Bolingbroke, who immediately denounced him to Richard as a traitor. Mowbray denied the charges and, as the two men were about to decide the dispute by duel, Richard intervened and banished them both (Sept. 16, 1398). Norfolk died in Italy shortly before Bolingbroke forced Richard to abdicate.