He became private secretary to Sir Robert Walpole. In 1739 was appointed Chief Secretary for Ireland by the Lord Lieutenant, William Cavendish, 3rd Duke of Devonshire; being chosen Member of Parliament for the borough of East Looe in 1740, and for Orford, Suffolk, at the general election in the succeeding year. Legge only shared temporarily in the downfall of Walpole, and became in quick succession Surveyor-General of Woods and Forests, a Lord of the Admiralty, and a Lord of the Treasury. In 1748 he was sent as envoy extraordinary to Frederick the Great, and although his conduct in Berlin was sharply censured by George II, he became Treasurer of the Navy soon after his return to England. In April 1754 he joined the ministry of the duke of Newcastle as chancellor of the Exchequer, the king consenting to this appointment although refusing to hold any intercourse with the minister; but Legge shared the elder Pitt's dislike of the policy of paying subsidies to the landgrave of Hesse, and was dismissed from office in November 1755. Twelve months later he returned to his post at the exchequer in the administration of Pitt and the 4th Duke of Devonshire, retaining office until April 1757 when he shared both the dismissal and the ensuing popularity of Pitt. When, in conjunction with the Duke of Newcastle, Pitt returned to power in the following July, Legge became chancellor of the exchequer for the third time. He imposed new taxes upon houses and windows, and the king refused to make him a peer. In 1754 Legge took the additional name of Bilson on secondarily succeeding to the West Mapledurham estate in Buriton near Petersfield, Hampshire of his cousin, Leonard Bilson MP upon the death of the original heir, Thomas Bettesworth, without issue. In 1759 he obtained the sinecure position of surveyor of the petty customs and subsidies in the port of London, and having in consequence to resign his seat in parliament he was chosen one of the members for Hampshire, a proceeding which greatly incensed the earl of Bute, who desired this seat for one of his friends. Having thus incurred Bute's displeasure Legge was again dismissed from the exchequer in March 1761, but he continued to take part in parliamentary debates until his death at Tunbridge Wells in 1764. Pitt called Legge, the child, and deservedly the favourite child, of the Whigs. Horace Walpole said he was of a creeping, underhand nature, and aspired to the lion's place by the manoeuvre of the mole, but afterwards he spoke in high terms of his talents. He "was a person of great abilities, both as a statesman and financier, and went through most of the great offices of government with reputation and integrity, and quitted them to the great regret of the nation in general."
