Metal Slug 2
Seeking to conquer our world, the Mars People have made an alliance with the Rebel Army, providing them with alien technology to help them on their coup. However, they planned to dispose of them once they had accomplished their goal.
As the Regular Army soldiers make their way into Donald Morden's base in the north pole, the Mars People betray him, killing his soldiers and abducting him, trapping him inside the Dai-Manji. The Regular soldiers fight the UFO, but it attaches itself into the Rugname, the martian's mothership. The Dai-Manji is hit by a kamikaze soldier, so before it could start a chain reaction they detach it and flee for the time being, leaving Morden behind.
Metal Slug 3
They would return again, abducting Morden and leaving an impostor to control his Rebel forces. When the Regular Army soldiers defeat the impostor, it reveals itself and captures one of them, taking them to the Rugname in space.
The Rebels help the Regular soldiers by launching rockets to reach the mothership. Their combined forces fight the Mars People inside the Rugname, defeating Rootmars, which causes the mothership to start self-destructing. They succesfully rescue Morden and their comrade, but Rootmars escapes and confronts them on their descent to Earth's atmosphere, being defeated and falling to the bottom of the ocean.
Metal Slug 6
The Mars People once again return, again allied with Morden's forces, but as it turned out, they were trying to fend off the Venusians, which feasted on them and would soon take over Earth. The three groups make a new alliance to fight against the new threat, defeating them after killing their queen.
Afterward, whenever the Rebel Army and Morden are under grave danger, the Mars People will always save them, as they are recently seen rescuing Morden's soldiers with their UFOs after the Regular Army soldiers defeated the Kraken, the experimental weapon Worm Mecha Morden modeled after.
The Mars People have appeared as fighters in SNK Vs Capcom: CHAOS, and later in Neo-Geo Battle Coliseum. They have also appeared as cards in the Card Fighters series.