After attaining his majority, he was returned on his father's interest for Scarborough in 1818. However, in the summer of 1819, he began to break with his family's Tory politics, and signalised his conversion to the Whigs by joining Brooks' Club on 3 December. When Parliament was dissolved in 1820, Normanby was in Florence, Italy, to which he was a regular visitor. His brother Charles kept up the family interest with the Scarborough corporation, and Normanby was returned in absentia in March, despite being politically at odds with his father. This state of affairs was not to last long: in May, Normanby was compelled to take the Chiltern Hundreds by Lord Mulgrave, to vacate the seat for Mulgrave's brother Edmund. His standing as a former Tory minister's son made Normanby valuable to the Whigs, and they hoped to return him to Parliament in another seat. An attempt was made to have him put in at St Ives at a by-election in 1821, but support proved to be lacking, and Normanby withdrew without contesting the seat. The illness and death of William Plumerin the beginning of 1822 allowed him to take his seat in February for Higham Ferrers, a pocket borough of the Whig grandee Earl Fitzwilliam. He made a considerable reputation by political pamphlets and by his speeches in the house. He was returned for Malton at the general election of 1826, another one of Fitzwilliam's boroughs. He was already known as a writer of romantic tales, The English in Italy ; in the same year he made his appearance as a novelist with Matilda, and in 1828 he produced another novel, Yes and No. He declined to be nominated again for Malton in 1830, anticipating the imminent death of his father, and was thus out of Parliament when Lord Grey formed a government in November 1830. Normanby hoped for employment by the foreign office, but none was forthcoming. Through Lord Durham, Normanby solicited a writ in acceleration from Grey in early 1831, which would have brought him up to the House of Lords before his father's death; but Normanby succeeded to the Earldom of Mulgrave on his father's death in April, rendering it moot. In 1832, Mulgrave was sent out as Governor of Jamaica and was afterwards appointed Lord Lieutenant of Ireland. On his visit to Wexford in 1836 he heard a Congratulatory Address in the ancient Forth and Bargy dialect, then already on the point of becoming extinct. He was created Marquess of Normanby on 25 June 1838, and held successively the offices of colonial secretary and home secretary in the last years of Lord Melbourne's ministry. While Colonial Secretary, he wrote a letter of instructions to William Hobson, in which the government's policy for the sovereignty of New Zealand was set out.
Diplomatic career
From 1846 to 1852 he was ambassador at Paris, and from 1854 to 1858 minister at Florence. The publication in 1857 of a journal kept in Paris during the stormy times of 1848, brought him into violent controversy with Louis Blanc, and he came into conflict with Lord Palmerston and William Ewart Gladstone, after his retirement from the public service, on questions of French and Italian policy.
Family
Lord Normanby married Maria Liddell, daughter of Thomas Liddell, 1st Baron Ravensworth, in 1818. He died in London on 28 July 1863, aged 66, and was succeeded in his titles by his son George. The Marchioness of Normanby died in October 1882, aged 84.