He was the son of Sir John Guildford (1430–1493), Comptroller of the Household to Edward IV, by his first wife, Alice Waller. He was relied on as a councillor by Reginald Bray, who chose him as one of the four persons to whom he first communicated the plot behind Buckingham's rebellion against Richard III in 1483. Both father and son raised forces that year for the Earl of Richmond (the future Henry VII) in Kent, and were attainted in consequence. The son, who thereby forfeited some lands in Cranbrook, fled to Richmond in Brittany, and returned with him two years later, landing along with him at Milford Haven, where he is said to have been knighted. It is presumed he was with Henry at the Battle of Bosworth. Little more than a month later, on 29 September 1485, the new king appointed him one of the chamberlains of the receipt of exchequer, Master of the Ordnance and of the Armouries, with houses on Tower Wharf, and keeper of the royal manor of Kennington, where the king took up his abode before his coronation.