He was a friend of Captain Avery Vaccares of the People's Navy. Holding the rank of Colonel in 1912 PD, he served as an interrogator at StateSec headquarters in Nouveau Paris. On orders from Secretary of State Security Oscar Saint-Just, he took the enigmatic arms dealer known as Charles aboard the captured Manticoran heavy cruiser HMS Ellipsis to implement a plan to fool the Anderman Empire into a war with the Star Kingdom of Manticore.
The plan eventually led them aboard the Andermani superdreadnought IANS Derfflinger, flagship of Großadmiral Chien-lu Anderman, Herzog von Rabenstrange. The plan was close to completion when the Großadmiral was tipped off by a deliberate glitch in the manipulated image from the "Manticoran" enemy ship's bridge. Realizing that Charles had betrayed him, Mercier tried to take Rabenstrange hostage, but was shot and killed by the Herzog's Totenkopf Hussars. (HHA5.2: AAoW)