Sam Guthrie is a mutant whose latent superhuman abilities first manifested themselves when he was trapped with co-workers in a coal mine. The stress of trying to save his partner and himself triggered his propulsive power and enabled them to escape. Donald Pierce, a renegade member of the Inner Circle of the Hellfire Club, located Guthrie and learned of his superhuman powers by means of a machine the Hellfire Club had built by adapting plans they had stolen from Professor Xavier's Cerebro device. Pierce recruited Guthrie to be one of his operatives, and sent him into battle against Xavier and his newly organized group, the New Mutants. However, during the battle, he refused to obey Pierce's order to kill the New Mutants, and Pierce attempted to kill Guthrie in retaliation. Xavier, however, saved Guthrie's life by defeating Pierce. Realizing that Guthrie was not an evil person, Xavier invited him to join the New Mutants, and Cannonball rose to become one of the New Mutants' leaders.
When the mutant warrior from the far future, Cable, reorganized the New Mutants into X-Force, Cannonball became a member of the new team. On one of X-Force's first missions, Cannonball was apparently slain in combat by Sauron, a member of the Toad's new Brotherhood of Evil Mutants. However, Cannonball soon returned to life and full health. Cable theorized that Cannonball was one of the Externals, a small number of immortal mutants. Whether or not Cannonball really is an External remains to be seen.
Recently, Sam graduated from his leadership role on the mutant team, X-Force, to the X-Men.
Cannonball possesses the ability to bodily generate thermo-chemical energy and release it from his skin. This energy, released by an act of will, is accompanied by smoke, flame and condensation, much like the exhaust of a rocket engine. At his present level of experience, he can only release this energy in one direction, beneath him. The equal and opposite reaction to this thrust causes his body to be propelled through the air like a human rocket.
At first, he could not stop his propulsion until his immediate store of energy was depleted or until he hit an obstruction of sufficient mass to stop him. However, he has since learned how to cut off his propulsive power at will. As a side effect of forming the thermo-chemical energies over the surface of his body, Cannonball becomes virtually invulnerable. Accompanying the release of energy is a half-inch thick energy field which channels the explosion and protects his skin from the direct effects of the blast. It also negates momentum and related energy effects thereby cushioning his entire body from any impact up to a half-minute from the depletion of his energy. Not only is he protected from rough landings, but he is also protected from nearly all types of injury while in flight. The protective field also protects any person or object with which he is in physical contact.
Cannonball once recovered from mortal injuries inflicted by Sauron that allegedly left him temporarily dead. It has been theorized that Cannonball's miraculous recovery was an indication that he was an External, a mutant gifted with immortality. It is unclear whether or not Cannonball is indeed an External, but it has subsequently been demonstrated that even Externals can be permanently killed.