[2] In 1893, he became professor of international law at the Free University of Brussels and two years later was elected to the Belgian Senate as a member of the Socialist Party.

He proposed a number of possible members, including Joseph Hodges Choate, Elihu Root, Charles William Eliot, and Andrew Dickson White.

[3] La Fontaine also promoted the idea of unification of the world's pacifist organizations.

[4] He was a member of the Belgian delegation to the Paris Peace Conference in 1919 and to the League of Nations Assembly (1920–21).

[5] Henri La Fontaine was a freemason, and a member of the lodge Les Amis Philanthropes in Brussels.