Before joining the X-Men, Calvin Rankin from Earth-12 tried out being a member of the Brotherhood of Evil Mutants. Their first mission was a failure, though, and the Mimic found himself imprisoned and abandoned by his teammates. When Professor Xavier offered to get him out of jail, Calvin reluctantly accepted. Soon, he befriended the members of the X-Men and eventually became team leader. The Mimic was counted among his world’s most popular super-heroes, and thanks to him, anti-mutant hysteria was at an all-time low. In short, Earth-12’s Mimic lived up to his full potential. But it was all taken away when he was plucked from his timeline to become a member of the reality-hopping Exiles, tasked with fixing critical events that had gone wrong across timelines.
(More information can be found in the Spotlight On... Mimic)
In the reality depicted in What if (2nd series) #74, Cyclops and Havok were recruited by Sinister rather than Xavier. Mimic was a member of the original X-Men, along with Banshee, Beast, Angel and Marvel Girl. His team clashed against Cyclops and Havok’s group whilst trying to trying to recruit young Ororo Monroe.
In the Age of Apocalypse, Calvin was one of numerous unlucky people held captive by the Sugar Man. It is unknown if he ever manifested his ability to mimic other people’s powers. The evil scientist tortured Calvin for weeks as he exposed him to a genetically-engineered strain of the penitence plague. Once the testing was done, Calvin was left to die a painful death. While they were too late to rescue Calvin, the Sugar Man’s plan to use the disease on the general population was stopped by Nate Grey, Forge and Magneto.
There’s one Mimic in the multiverse who personifies both the best and the worst that Calvin Rankin can be. Also starting out as a member of the Brotherhood of Evil Mutants, this Mimic did not accept Xavier’s offer to join the X-Men in order to get out of prison. Eventually breaking out of prison on his own, Calvin assumed the alias “The Big M” and became the Brotherhood’s rather ambitious and effective leader. Under his command they performed many terrorist attacks on humanity. The Big M counted several thousand people among his victims, including Charles Xavier and all of the X-Men. However, when the Big M met the Mimic from the reality-hopping Exiles and telepathically experienced his counterpart’s memories, he had a change of heart. Vowing to never kill again, Calvin dedicated himself to Xavier’s dream of peaceful coexistence and re-opened his school to train mutants in the use of their powers.
When Magneto and his followers first established the rule of the House of M on the island of Genosha, the US government tried to contain the threat to the rest of the world with a covert operation. They sent in a small strike-force consisting of Nuke, Bucky and the Mimic. Upon arriving on the island, Calvin copied the abilities of dozens of mutants, including Magneto himself. He then engaged the Master of Magnetism in a one-on-one duel. Calvin had the clear advantage in terms of shear power. However, while Calvin offered Magneto the chance to surrender peacefully, Magneto magnetically summoned a power-negating collar and snapped it around Mimic’s neck. Powerless, he dropped out of the sky. Although he survived the fall, Mimic didn’t live much longer, as Magneto beat in his skull with a wooden pole.
Another version of Mimic, similar to the Exiles’ Mimic, appeared as a part of the Grandmaster’s Exiles. On their first mission, the Exiles inadvertently interfered in a wager between the Grandmaster and the Collector of that world. To exact revenge on the Exiles the Grandmaster travelled across numerous realities until he found a world that held exact analogues of all the Exiles, Earth 33629. Posing as the Timebroker, he gathered his Exiles together under the pretence they would be fixing reality when in actual fact they were just a trap to lure the real Exiles in. The two Exiles teams fought, until they realized they were being duped, at which point they turned on the Grandmaster. Once he was dealt with, Mimic and the rest of the alternate Exiles remained together, acting as the planet’s premier superhero team.
In the Ultimate universe, Calvin Rankin was a soldier who served in Afghanistan. Inspired by his childhood hero Captain America, Calvin volunteered to join his government’s super-people 
program. He was kept in a secret training facility in Bulgaria alongside other potential candidates hoping to become part of the United States’ premier super hero team, The Ultimates. The facility was raided by Chinese forces who stripped the place clean of every bit of technology they could use to boost their own program to create super-powered agents. Rankin apparently needed this technology to control his bodily functions, and his powers as Mimic flared up when the Ultimates arrived. He uncontrollably copied their powers and eventually died from the strain.