William Brewer, 1st Baron Brewer (alias Briwere, Brigwer, etc.) (died 1226) of Tor Brewer in Devon, was a prominent administrator and judge in England during the reigns of kings Richard I, his brother King John, and John's son Henry III. He was a major landholder and the founder of several religious institutions. In 1204, he acquired the feudal barony of Horsley in Derbyshire.
Brewer's ancestry is unclear, but he was probably the son of Henry Brewer and the grandson of William Brewer, Royal Forester of Bere, Hampshire, who founded the nunnery of Polsloe in Exeter. William Brewer, Bishop of Exeter, was one of his nephews.
He began his career as Forester of Bere, which appears to have been a hereditary title, and by 1179 had been appointed Sheriff of Devon. Under King Richard I (1189–1199) he was one of the justiciars appointed to administer the kingdom while the king was on the Third Crusade. He was present at Worms, Germany, in 1193 to aid in the negotiations for the ransom of King Richard. In about 1193 he began his career as a Baron of the Exchequer, an office that he exercised until the reign of King Henry III (1216–1273).
Under King John (1199–1216) Brewer was one of the most active figures in government, in terms of the number of royal charters he witnessed, together with Henry Marshal, Bishop of Exeter and Geoffrey FitzPeter, 1st Earl of Essex. During this period he was appointed Sheriff of Berkshire, Sheriff of Cornwall, Sheriff of Devon, Sheriff of Hampshire, Sheriff of Nottinghamshire and Derbyshire, Sheriff of Oxfordshire, Sheriff of Somerset and Dorset and Sheriff of Sussex and Sheriff of Wiltshire. He was often unpopular with the inhabitants of his counties, and the men of Cornwall, Somerset, and Dorset paid money to the king for his removal.