Arenberg served in the Chamber of Deputies from 1877 to 1881. He was elected as the official candidate of the MacMahon government, winning the poll due to the abstention of republican voters disenchanted with his predecessor. In the Chamber, he took his place among the members of the monarchist Right, voting consistently with the conservatives, including against the legalization of divorce.
Returned to parliament as a monarchist candidate again in 1889, Arenberg continued his opposition to republican government. After Rerum novarum and Pope Leo XIII's recognition of the Third Republic, however, Arenberg recast himself, running in 1893 as a "liberal republican". In the Chamber, he concentrated on colonial issues, in particular those concerning Africa; among his projects were securing free navigation of the Niger River and delineating Anglo-French colonial boundaries.
Defeated in the 1902 elections and failing to secure reelection again in 1906, Arenberg retired from politics, but remained active in public life. He was the first president of the procolonial Comité de l'Afrique française and remained active in the organization until his death. A convinced Catholic himself, in 1895 he was one of the organizers of a failed attempt to build a mosque in Paris through private donations. From 1896 he was also president of the Compagnie universelle du canal maritime de Suez, and he was a member of the Institut de France (Académie des beaux-arts) from 1897.