A lawyer, journalist, and teacher of political economy, Courtney was elected to the House of Commons in 1876. Appointed financial secretary to the treasury by Prime Minister W.E. Gladstone in 1882, he resigned in 1884, when the Cabinet refused to include proportional representation in a parliamentary reform bill. In 1883 he married Kate Potter, whose sister and brother-in-law were the Fabian Socialists Beatrice and Sidney Webb.

Courtney had become a constant critic of British imperial expansion in Africa since 1877, when the Transvaal was annexed. In 1900 he lost his seat in the Commons because of his condemnation of the South African (Boer) War of 1899–1902. Although he originally (1886) opposed Gladstone's plan for Irish self-government, he was converted to the idea of Home Rule after he had come to see Anglo-Irish relations in terms of British imperialism. From about 1908 he attacked British foreign policy as likely to provoke a European war, and during World War I he advocated a negotiated peace and fearlessly defended conscientious objectors. In 1906 Courtney was raised to the peerage. His marriage was childless, and the barony lapsed upon his death.