Newcastle was returned to Parliament for South Nottinghamshire in 1832, a seat he held until 1846, and then represented Falkirk Burghs until 1851, when he succeeded his father in the dukedom. Initially a Tory, he served under Sir Robert Peel as First Commissioner of Woods and Forests from 1841 to 1846 and as Chief Secretary for Ireland in 1846, as the effects of the Great Irish Famine began to take hold. He was admitted to the British Privy Council in 1841, and to the Irish Privy Council on 14 February 1846.