After studying law and Oriental languages at the Lycée Louis-le-Grand in Paris, Estournelles de Constant embarked on a diplomatic career in 1876.

Among Estournelles de Constant's early diplomatic posts were Montenegro, the Ottoman Empire, the Netherlands, Great Britain, and Tunisia; in 1882 he returned to Paris to serve as assistant director of the Levant bureau of the Ministry of Foreign Affairs.

In 1890 he was posted to London as the French chargé d'affaires, where he played a role in averting war with Britain over colonial disputes.

In 1899, then in 1907, Paul d'Estournelles de Constant represented France, with Léon Bourgeois and Louis Renault, at the Hague Conferences.

He helped Léon Bollée, the main supporter of the American Wilbur Wright, an aviation pioneer, in his aeronautical experiments carried out between August 8, 1908 and January 2, 1909, at Le Mans and in Sarthe, on the Hunaudières racecourse then at the Auvours military camp.

Paul d'Estournelles de Constant receives the Nobel Peace Prize on December 10, 1909, jointly with the Belgian deputy Auguste Beernaert, for their efforts in the construction of international law, in particular in the organization of the conferences of The Hague of 1899 and 1907 which lead to the creation of a Permanent Court of Arbitration17.