[2] His parents were Antoine Joseph Eugène Deloncle (1829–1887) and Anne Madeleine Adèle Caroline Joséphine Lathelize (born 1832).

[3] His elder brother Louis was a sea captain who was killed when his ship, the ocean liner La Bourgogne, sank in 1898.

[4] Eugène Deloncle was a university professor who had resigned after the 2 December 1851 coup d'état and had been deported to Algeria for refusing to take the oath.

[5] Deloncle then travelled in Algeria and Tunisia before studying at the École des langues orientales from 1873 to 1877, where he was appointed substitute professor of Hindustani.

[5] Deloncle was appointed secretary to the Bucharest embassy in 1881, then returned to assist in the trade negotiations with Italy, Switzerland and England.

[6] On 14 November 1881 Léon Gambetta, President of the Council and Minister of Foreign Affairs, named Deloncle Chief of Staff to Eugène Spuller, Under-Secretary of State.

His dispatches were embellished and enlivened by Octave Mirbeau and published under the pseudonym "Nirvana" in 11 instalments in Le Gaulois between 22 February and 22 April 1885.

[5] On 22 September 1889 Deloncle ran as Opportunist Republican candidate for the Castellane constituency of Basses-Alpes, and was elected in the first round by 2,391 votes out of 4,265.

[11] Deloncle was involved debates over the French settlements in Dahomey, the old treaties between France and the Sublime Porte of the Ottoman Empire, customs duties on maize and rice, the Sultanate of Zanzibar, the June 1890 commercial convention between China and England, the general tariff, the Brussels Conference Act of 1890, French colonization of Madagascar, events in Dahomey, the 1885 Monetary Agreement with Greece, Switzerland, Italy and Belgium, creation of a vice-consulate in Muscat, liquidation of the Universal Company of the Panama Interoceanic Canal, creation of diplomatic posts in Fez, Sierra Leone, Han-Kieou and Fort Dauphin.

[12] Deloncle and Émile Flourens, both senior members of the groupe colonial, denounced "English dreams of African hegemony" while calling for a rapid advance by France to the Upper Nile.

"[13] He obtained control of the Journal Egyptien which he turned over to Aristide Gavillot, a wealthy French journalist and member of the Egyptian administration whom the British saw as a puppet of Deloncle.

He was a member of the Committees on Associations and Congregations, External Affairs, Protectorates and Colonies, Revision of the Civil Pensions Act of 1853 and Public Works.

He spoke on approval of the 13 August 1902 commercial convention concerning the Dutch colonies, funding of the South Pole Scientific Expedition, protection of French interests in the Baghdad Railway Company and defense of Indochina.

He was rapporteur for the bills to approve the 11 January 1902 agreement with Montenegro for protection of literary and artistic works, the 6 April 1904 convention with the Netherlands concerning submarine cables landing in the Netherlands Indies, the 13 February 1904 convention between France and Siam and the 15 February 1904 agreement with the Eastern Extension Australasia and China Telegraph company sale to the French Government of the cable from Saigon to Haiphong.

[1] In a by-election of 28 April 1912 to replace Justin Perchot(fr), who had become a senator, Deloncle was elected deputy for Castellane, Basses-Alpes, in the second round of votes on 5 May 1912.

He was involved in discussions on foreign policy, events in the East, the Balkan Wars and reestablishing official diplomatic relations with the Holy See.