Mercer was an ex-Cobra Viper who joined the Joes, starting out as a member of Sgt. Slaughter's Renegades, a trio of probationary Joes who did mostly black-ops work and do not officially "exist".
Marvel Comics/Devil's Due History
Mercer debuts in the Devil's Due issues of the G.I. Joe comic series, where he leads an assault team on Cobra Island during the second Cobra Civil War. His team, comprised of himself, Jinx, Sci-Fi, new member Hacker, Mainframe, and Flash, is assigned with the task of taking out the revived Serpentor's EMP weapons. Mainframe and Flash are separated from the rest of the squad, and when Hacker is badly injured, Mercer leads a retreat while the two missing Joes managed to destroy the EMP generators. Mainframe and Flash don't make it out alive because of interfering Cobra forces.
Mercer is later one of the Joes kept on the team after the Jugglers place General Philip Rey in charge. After the team is reorganized as a smaller unit under General Joseph Colton, Mercer served as a reservist. In G.I. Joe: Special Missions vol. 2 #1, he was called into duty to aid a group of other reserve Joes (Beachhead, Cover Girl, Low-Light, and Tunnel Rat) in securing an abandoned Cobra bioweapons facility in Manhattan. While the other Joes storm the lab itself, Mercer battled Cobra operative Neurotoxin one-on-one. Mercer later returns to duty during the "World War III" story arc in G.I. Joe: America's Elite, where he is deployed to Iran, joining Dusty and Airtight in battling Cobra forces there.
G.I. Joe vs The Transformers
Mercer is featured in the first of Devil's Due's G.I. Joe/Transformers crossovers. He leads an attack on Cobra Island in order to shut down Cobra's alliance with the Decepticons. When an energy transfer device malfunctions, bombarding the island with energy beams, Mercer is incinerated in a random blast.
G.I. Joe: The Movie (1987)
He is featured in the Sunbow film as part of the Renegades. He is voiced by Kristoffer Tabori.
DIC's G.I. Joe cartoon (1989-92)
He appears in his V2 uniform in the second season episodes "The Eliminator" and "Kindergarten Commandos".
Mercer's first figure was released in 1987 as part of the "Sgt. Slaughter's Renegades" three-pack, along with Red Dog and Taurus. He came with a (exaggeratedly large) silenced pistol and a repaint of the character Footloose's backpack. His next figure was released in 1991, where he was depicted with a beard and a different name and birthplace on his filecard. He came with a backpack missile launcher, as with most of the figures at the time. His next figure was released as part of the 2006 G.I. Joe Collector's Convention's bonus exclusive figures, which supplemented the boxed set. He was carded with his fellow Renegades, and the three-pack was part two of the bonus set, which included Sgt. Slaughter. His figure was made with a retooled version of his original head sculpt, but like Red Dog and Taurus, used the arms of the 1992 Gung-Ho and the torso, waist, and legs of the Mega Marines Clutch. His most recent figure was a 'modern era' (25th Anniversary style) figure in the Big Bad Toy Store-exclusive "Slaughter's Marauders" 7-pack. It was made up of a new head sculpt, the torso and upper legs of the Pursuit of Cobra Cobra Shock Trooper, the lower legs of the Pursuit of Cobra Zartan, and the arms of the Rise of Cobra Arctic Threat Storm Shadow. He comes with a recolored version of the 25th Anniversary Alley-Viper's vest (complete with knife for the sheath), the 25th Anniversary Viper's rifle (as a homage to his days in that group), and a silenced pistol from the third Pursuit of Cobra Snake Eyes figure (as a homage to the original figure). He was also renamed "Mercenary" due to trademark reasons. His filecard also lists his birthplace as Taos, New Mexico (the same as Spirit's) by mistake due to a factory error.