Derby entered Parliament for Westhoughton in 1892, and served under Lord Salisbury as a Lord of the Treasury between 1895 and 1900 and under Salisbury and later Arthur Balfour as Financial Secretary to the War Office between 1901 and 1903. In October 1903 he entered the cabinet as Postmaster General, a post he held until the government fell in December 1905. He lost his seat in the 1906 general election. In 1908 he succeeded his father in the earldom and took his seat in the House of Lords.