Gray Mann is the third son of Zepheniah Mann. The smallest of the three Mann brothers, Gray was also the most intelligent of the three. He could speak perfectly by the time he was born, but was the weakest physically. Disgusted by how he could talk, Zepheniah Mann was going to smother Gray, but before his father could do so, Gray was abducted by a wild eagle; the event taking place during the "Great Eagle Scourge of 1822". The eagle took Gray away from his family, and raised as one of its own.

After being raised by his eagle abductor, Gray killed and ate her and her children before crawling back to civilization to build his corporate empire, Gray Gravel Company. According to Zephaniah Mann, in 1849, Gray Mann had threatened to blackmail him for his cache of “miracle” gravel.

In 1971, Gray resurfaced, revealing that, like his brothers, he possessed a life-extending machine, albeit a more efficient and compact version mounted on his back. After luring them to a fake meeting using forged letters from each other offering a truce, he assassinated the Engineers operating Redmond and Blutarch’s life extending machines, then killed his twin brothers Blutarch and Redmond Mann to take control of their respective companies and absorb them into Gray Gravel Co. Gray created an army of robots to destroy Mann Co. facilities all over the world as shown in A Fate Worse Than Chess. Saxton Hale rehired the mercenaries, who had been fired from BLU and RED after the takeover, to defend these factories from the robots.

In 1972, Gray surrendered the war to Saxton Hale, acknowledging that his war had turned into a stalemate reminiscent of the Gravel War between his brothers. However, he used the Mann Co. Challenge policy, which stated that the CEO of any company could legally seize control of Mann Co. if they beat Mann Co.’s CEO in unarmed combat, in order to pit Saxton Hale against his daughter, Olivia. Unwilling to fight a child, Saxton Hale conceded, giving up control of the company to Olivia.

Six months later, Gray’s Australium-fueled Life Extender Machine began to run low. In desperation, Gray uncovered Mann Co.’s safe of Australium. He discovered it empty, and deduced that the Australium had been stolen by the after finding only a cigarette and a single brick of Australium left behind in the vault. He proceeded to hire an unknown group of individuals to find the and recover the Australium. In A Cold Day in Hell, it is revealed that Gray hired the Team Fortress Classic Mercenaries to find the . During the events of Old Wounds, Gray’s Life Extender Machine is ripped apart by the TFC Heavy, leading to Gray’s eventual death. Before ing away, Gray appeals to Miss Pauling and the mercs, claiming that the accumulated the Australium to enact a plot far more evil than his, and asks them to stop her.