Lieutenant-Colonel Gregory D. "Greg" Mallory is a WWII veteran, as well as the founder and original team leader of the Boys. He and Billy Butcher created the team to stop Vought-American's Supes from joining the United States Military, but after several losses and arguments with the rest of the team, Mallory ultimately left.
After witnessing firsthand the poor discipline and performance of superheroes during the Battle of the Bulge (which resulted in his entire platoon being wiped out), Greg Mallory made it his life-long mission to stop Vought-American from incorporating superheroes into the United States Military. He thus joined the C.I.A., and would go on to recruit Billy Butcher after his wife Becky's death. Together, they founded the Boys, a C.I.A. black ops team meant to keep Supes in check. Mother's Milk, the Frenchman, and the Female were all eventually recruited.
During this time, Mallory and Butcher came into conflict more and more frequently over the direction of the team. Butcher eventually convinced Mallory to blackmail the Seven with photographs of Homelander (actually Black Noir) committing atrocities. This backfired, however, as the Lamplighter followed Mallory to his hotel and burned his grandchildren alive. As a peace offering, the Seven handed over Lamplighter, whom Mallory killed in revenge. Shortly after, Mallory retired and left the Boys in Butcher's hands, but stayed in contact with Mother's Milk to monitor things. Mallory eventually came into contact with Hughie, revealing the backstory of the Boys. After this, it is implied that he was killed by Butcher, although his fate was never shown.