Mercenaries are irregular combatants partaking in armed conflict in exchange for monetary compensation. A type of mercenary, the private security contractor, is a formal armed service offering services in exchange for payment.
In the Strangereal world, mercenaries were predominantly active in the late 20th century, with hired pilots seeing action in the Yuktobanian war against Romny in 1986, and later again in the 1987 Blood War. The 112th Tactical Fighter Squadron saw combat there under Yuktobania's command, suffering heavy casualties through the two wars. Dominic Zubov, one of the few pilots who was not driven insane by the action, became a mercenary in the Verusan continent before being hired by Belka in the next decade.
Several mercenaries were hired by the newly founded Ustio in 1995 in the opening of the Belkan War to replace the losses of their air force. One of these squadrons, the Galm Team, became a vital element in the expulsion of Belkan troops from Ustio and the Allied Forces' eventual victory, along with the suppression of the "A World With No Boundaries" army. In particular, Galm leader Cipher gained much fame in the frontlines, becoming a known figure between foe and friend alike.
Scarface One, an enigmatic mercenary with experience across the world, was hired along with several other pilots by the FCU to crush a coup d'etat in the Usean territory of Scali Island in 1995. The "Scarface Squadron" was disbanded after Scali, and Scarface himself retired from war. Two years later, in 1997, he returned to action in service of Usean forces opposing a continental revolt against the Osean Federation.
In the world of Ace Combat: Joint Assault, many mercenaries were hired by Andre Olivieri to carry out his "Golden Axe" plan, which comprised the Golden Axe Plan Private Army. A PMC company, Martinez Security, fought and stopped Oliveri's army in clandestine operations in the United States, culminating in a decisive battle between the Antares Squadron and the GAPPA, commanded by former Martinez pilot Milosz Sulejmani and his Varcolac unit.
Within the Horizon universe, mercenaries from Russia were hired by the Blatnoi organized crime group with the goal of instigating a regional revolt in East Africa, who aided the local rebels with military training. Andrei Markov, the leader of the New Russian Federation's air force, left the Russian Air Force and became a freelance pilot sometime before the 2015 coup.
In the world of Ace Combat Infinity, the global military budget cuts caused by the Ulysses 1986VG1 catastrophe triggered a boom in the private security contractor industry. Several armed groups were hired by the United Nations Security Council, forming the UNICS (United Nations Independent Commando Services). As well, the overabundance of aircraft after the introduction of the Advanced Automated Aviation Plant technology caused several formal pilots to become hired soldiers, giving rise to groups such as Martinez Security, Axe & Hammer and Arrows Air Defense and Security.