Lafontaine studied law at the Free University of Brussels. He was admitted to the bar in 1877 and established a reputation as an authority on international law. In 1893 he became professor of international law at the New University in Brussels and two years later was elected to the Belgian Senate as a member of the Socialist Party. He served as vice chairman of the Senate from 1919 to 1932.

Lafontaine took an early interest in the International Peace Bureau, founded in 1882, and was influential in the Bureau's efforts to bring about The Hague Peace Conferences of 1899 and 1907. He was a member of the Belgian delegation to the Paris Peace Conference in 1919 and to the League of Nations Assembly (1920–21). In other efforts to foster world peace, he founded the Centre Intellectuel Mondial (later merged into the League of Nations Institute for Intellectual Co-operation) and proposed such organizations as a world school and university, a world parliament, and an international court of justice.

Lafontaine was the author of a number of legal handbooks and a documentary history of international arbitration. He was also founder of the review La Vie Internationale.