A lawyer from 1854, Harcourt briefly taught international law at the University of Cambridge. Entering the House of Commons in 1868, Harcourt served Prime Minister William Ewart Gladstone as solicitor general (1873–74), home secretary (1880–85), and chancellor of the Exchequer (1886, 1892–94); he was knighted in 1873. On Gladstone's retirement (March 1894), he was a leading but an unsuccessful candidate to succeed him as prime minister. Although Harcourt remained as chancellor of the Exchequer in the 5th Earl of Rosebery's government (1894–95) and became leader of the House of Commons, he was not on friendly terms with Rosebery and, for reasons of policy and personality, drifted away from the other Liberals. After resigning as leader of the Liberal opposition in the Commons (1898), he attacked British imperialism in the Transvaal and, later (1903), the protective-tariff proposals of Joseph Chamberlain.

By imposing a graduated tax on the total estate of a deceased person, Harcourt's legislation of 1894 was capable of producing much more revenue than taxes only on the amounts inherited by beneficiaries. The new death duties were enacted over the opposition of Rosebery and Gladstone, who believed that easily increased taxes would encourage frivolous governmental spending. Other opponents regarded the tax as an attack on the great hereditary landowners.