Three teenage bad seeds - Gizmo, Jinx and Mammoth - were about to graduate from the H.I.V.E. Academy and were looking to be employed by criminal patron Slade. Slade send them out to destroy the Teen Titans, ostensibly as a graduation test, but actually to surreptitiously (more or less) draw Robin's attention to him. The trio temporarily took over Titans Tower but were ultimately defeated by the Titans; as a result, they were forced to repeat their graduation period.
Later on, Brother Blood took over the Academy and sent his students out on robberies to finance the development of a new weapon. Cyborg infiltrated the Academy in disguise and eventually (and quite literally) brought it down. During his stay in the facility, he befriended Jinx, Gizmo and Mammoth, though this was broken up when Cyborg's true identity was revealed.
Following the destruction of the H.I.V.E. Academy, Mammoth and his two friends went freelance and eventually formed the nucleus of the H.I.V.E. Five; as such, he acted as the team's principal strongman and brawler. However, this did not save him from the influence of Mother Mae-Eye after the Titans had managed to trick them into taking in the pie she was imprisoned in, though the H.I.V.E. Five eventually escaped her control. Later on, Mammoth participated in the attempted capture of Kid Flash, but his immense strength proved no match for the speedster's abilities.
When the Brotherhood of Evil decided to eliminate young superheroes worldwide, Mammoth, Jinx and Gizmo were recruited into their ranks, along with a large number of other villains, most of them old enemies of the Titans. While Gizmo and Billy Numerous attacked Kole and Gnarrk in their arctic underground retreat, Mammoth intercepted Cyborg, who had been sent to aid them, slamming him down a crevasse. However, he did not bother to finish his job properly, and so Cyborg was able to collect Kole and Gnarrk and join his fellow Titan refugees in an all-out assault on the Brotherhood's base. When the H.I.V.E. Five attempted to escape the scene, they were intercepted by Kid Flash and Jinx, who had since converted to good, defeated by a hex blast and carted off to be flash-frozen by Mas y Menos.
Powers and Abilities
Mammoth is the power house of the H.I.V.E. and is many times stronger than the average human being. The exact range and limitations of Mammoth's strength level are unknown, though it appears to be on a scale equal to (or even surpassing) Cyborg's. Complementary with his size and strength, Mammoth is extremely tough and can withstand great physical extremes without tiring.
In addition, Mammoth possesses a perpetual appetite (also much like Cyborg) and can consume mass quantities of food (fresh or spoiled) with no ill effect.