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. Frustrated by the limitations of diplomatic service, he ran for parliament in 1895, securing a seat in the Chamber of Deputies. In 1904 Estournelles de Constant ran for and won a seat in the Senate, where he served until the end of his career in 1924.
As a deputy and senator, Estournelles de Constant concerned himself with colonial issues, consistently opposing Third Republic colonial policy. He advocated the elimination of colonial seats in the French parliament, preferring a policy of establishing protectorates to the traditional republican programme of colonial assimilation. In particular, he violently opposed the establishment of French colonial rule in Madagascar and the Great Powers' dismemberment of China. In domestic affairs, he was concerned particularly with what the terminology of the day called "outrages against morality" (outrages aux bonnes mœurs).
On the other hand, he was a Dreyfusard and argued in favour of placing Émile Zola's remains in the Panthéon for Zola's part in the Dreyfus Affair.
Above all, though, Estournelles de Constant dedicated himself to the cause of improving international relations, and he was a member of the Permanent Court of Arbitration from 1900. He represented France at both Hague Peace Conferences (1898 and 1907), and outlined a vision of a European union.
Estournelles de Constant wrote historical and political works and even dabbled in playwriting. In addition, he was a regular contributor to the newspapers Le Temps, La Revue de Paris, and La Revue des deux mondes. Married to an American, Daisy Sedgwick-Berend, he also travelled extensively in and wrote about the United States.
Estournelles de Constant's name may be encountered in numerous variants; the article title reflects the form standardized by the Bibliothèque nationale de France.