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.

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.

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.

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.