Cosmao was the third and final member of the Troude class of protected cruisers built for the French Navy in the late 1880s and early 1890s.

The class was built as part of a construction program intended to provide scouts for the main battle fleet.

They were based on the preceding Forbin class, the primary improvement being the addition of armor to the conning tower.

Cosmao served in the Mediterranean Squadron through most of the 1890s, where she was primarily occupied with training exercises with the rest of the unit.

Tasked with patrolling for German U-boats and protecting Allied merchant shipping, Cosmao saw no combat during the conflict.

After the first two ships were ordered, the navy requested competing proposals from private shipyards, and the design from Forges et Chantiers de la Gironde was selected, which became the Troude class.

The ship's propulsion system consisted of a pair of compound steam engines driving two screw propellers.

Work on Cosmao began with her keel laying at the Forges et Chantiers de la Gironde shipyard in Lormont in 1887.

She was then assigned to the Escadre de la Méditerranée (Mediterranean Squadron), the main French battle fleet., departing Rochefort for Toulon on 14 August.

[4][7] She returned to service with the unit that year, by which time the fleet's cruiser division consisted of Cosmao, her sister ships Lalande and Troude, Tage, and Suchet.

The first consisted of squadron exercises and shooting practice, the second included a cruise between several French ports in the Mediterranean, and the third centered on a simulated battle between elements of the fleet organized into three separate divisions.

[12] The annual maneuvers were conducted in July, and during the exercises, Cosmao failed to detect the ships of the Reserve Squadron, which were serving as the simulated enemy fleet.

They were also responsible for escorting convoys and patrolling anchorages in the Canary Islands to ensure German U-boats were not using them to refuel.

Her armament was reduced again in 1918, and in October she was sent to patrol the Syrian coast in the Levant Station through the end of the war in November.