On his return to England Cary was introduced to court, and became Gentleman of the Bedchamber. He was knighted at Dublin in 1599. In 1601 he was elected Member of Parliament for Hertfordshire. He was a JP for Hertfordshire in 1601. He became joint master of the jewels with his father on 21 June 1603. In 1604 he was re-elected MP for Hertfordshire. At the investiture of Charles Prince of Wales in 1616 he was created a Knight of the Bath In 1617 he became Comptroller of the Household and a Privy Councillor. He succeeded to the family estates on the death of his father in 1618. He was created Viscount Falkland in the county of Fife, in the Scottish peerage on 10 November 1620 (the title, with his naturalisation, was confirmed by Charles I by diploma in 1627). In 1621 he was re-elected MP for Hertfordshire; his Scots peerage gave him the right, which he was the first to exercise, of sitting in the English Commons.