He was created a Knight of the Bath (KB) in 1610 and sat in the House of Commons as the member for East Retford in the Addled Parliament of 1614. He succeeded his father in 1617.

On 3 November 1620 Cavendish was created 'Viscount Mansfield', then 'Earl of Newcastle-upon-Tyne' in 1628. In 1629 he inherited his mother's barony of Ogle, together with an estate of £3,000 per annum. In 1638 he became governor of Charles, Prince of Wales, and in 1639 a Privy Counsellor. When the Scottish war (1639–1640) broke out he assisted King Charles I with a loan of £10,000 and a troop of volunteer horse, consisting of 120 knights and gentlemen. He was appointed Gentleman of the Robes in 1641, but was implicated in the Army Plot, and in consequence withdrew for a time from the court.