Trained as an equity and conveyance lawyer, Westlake helped establish the Working Men's College, London, in 1854 and was one of the founders of the Institut de Droit International (Institute of International Law (International Law, Institute of)) in 1873. He was a Liberal member of Parliament (1885–86) and Whewell professor of international law at the University of Cambridge (1888–1908). Among the social reforms for which he fought was woman suffrage; he also worked actively for the restoration of the constitution of Finland (1899–1900).

Westlake's Treatise on Private International Law (1858) was a pioneering work in the field as practiced in England, and it exercised profound influence on numerous subsequent judicial decisions. His other works include International Law (part 1, Peace, 1904; part 2, War, 1907). His Collected Papers were published in 1914.