Irish nobleman and British diplomat who served as viceroy of Canada and of India, secretary for war, and foreign secretary.

The eldest son of the 4th Marquess, he attended Eton and on the death of his father succeeded, at age 21, to the marquessate and great lands and wealth. Joining the Liberal Party he was a lord of the Treasury (1868) and under secretary for war (1872–74) and for India (1880). As governor general of Canada (1883–88) he effected an agreement with rebelling Indians and used his French language ability to facilitate acceptance.

Conservative Prime Minister Salisbury appointed him viceroy of India, and his administration (1888–94) was marked by peace except for a short rising in the independent state of Manipur, for which the leader Tikendrajit was executed. Lansdowne founded an imperial library and record office, abolished the presidential army system, closed Indian mints to the free coinage of silver, reorganized the police, reconstituted legislative councils, gave council members rights of financial discussion and interpolation, and extended railway and irrigation works. The independent kingdom of Sikkim was brought under British protection in 1888 and its boundary with Tibet demarcated; Hunza and Nagar on the Afghan frontier were annexed in 1892.

Lansdowne became secretary of state for war in 1895, and charges of unpreparedness for the South African War brought demands for his impeachment in 1899. After the 1900 elections the Conservative government remodelling brought him in as foreign secretary (1900–06) amid protests. In 1906–10 he was leader of the minority Conservative opposition in the House of Lords and deplored the disparity of parties there. He was minister without portfolio (1915–16) in Asquith's government. His controversial published “Lansdowne Letter” (1917), calling for a statement of intentions from World War I Allies, was criticized as contrary to public policy.