Calvin Rankin was born the son of a prominent scientist. His father was involved in certain experiments while working with the Canadian government, experiments which ultimately led to the early days of the Weapon X Program. Ronald Rankin had carried out some of his work using himself as a guinea pig. As a result, Cal was born with an inherent capacity for mutation. [Marvel Comics Presents (1st series) #59]

After accidentally upsetting one of his father's experiments, Cal was exposed to chemical vapors which activated within him the power of mimicry. As he grew of age, Cal Rankin came to master the strengths and skills of everyone he came near - he excelled in all his classes since his knowledge matched that of his teacher, and his athletic ability became the combination of the best traits of everyone nearby, making him a star on every sports team in his school.

This gift made Calvin grow conceited and his attitude began to make enemies out of his classmates. When his father became aware of Cal's power, he moved his son and his laboratory to a hidden cave. He hoped to devise a means of removing his son's power before mankind turned against him for being different. To secure Cal's help, Ronald Rankin lied and told him the experiments were to make his mimicked skills permanent, instead of fading when he got out of range. Ronald's laboratory drew a great deal of electrical power from the nearest city while building his mimicry machine and the townspeople eventually tracked the resulting brownouts to the cave. Fearful of what the mob might do to him and his son, Ronald planned to detonate a series of explosives and block the cave's entrance from attackers. Tragically, he misjudged the explosive blast and died before he could get a safe distance away.

Cal became tremendously bitter towards the world following his father's death. He swore to take revenge on humanity once he found a way back into the cave laboratory, where he hoped to use his father's machines to augment his powers. Months passed, and Calvin resumed living among regular people again until he could devise a means of reaching the buried machine. While researching mine engineering at the Salem Center library, he grew infatuated with the librarian, Vera Cantor, and kept pressuring her for a date.

One day, he confronted Vera and her friend Zelda while they were on a double date with young Hank McCoy and Bobby Drake, AKA Beast and Iceman of the original X-Men. When Cal provoked a fight with the two boys, he suddenly found himself with ice-generating powers and superhuman strength and agility. The X-Men dared not reveal their powers in public. Witnesses to the fight believed Calvin was a mutant attacking two helpless teenagers and chased him off. After escaping, Cal quickly realized who he had been fighting and came up with a plan to use the X-Men's abilities to reopen his father's cave laboratory. Tracking them back to Xavier's School for Gifted Youngsters, he brazenly knocked on the front door and asked to join the team.

After absorbing the full team's powers, Calvin confronted the group in his new identity as the Mimic. After a short battle, he grabbed Marvel Girl and made his escape, daring the rest of the team to follow. Leading them to the site of his father's cave, Mimic was able to use their powers to tunnel his way straight into the old laboratory and access his father's machine. Engaging the team in battle again, the X-Men were called off by Professor Xavier, who allowed Mimic to expose himself to the machine. Just as his father had planned, Calvin's abilities were wiped clean and he was left powerless. After Xavier telepathically wiped the Mimic's recent memories, he was allowed to go free, none the wiser about the existence of the X-Men or their true identities. [X-Men (1st series) #19]

Months later, Calvin ran into Jean Grey on the campus of Metro College. An accident in the chemistry lab restored his memories and his powers. [X-Men (1st series) #26]
Though he was still as arrogant as ever, Cal chose to join forces with the X-Men, who were suspicious of their former foe. Cyclops had suffered a crisis of faith after injuring Angel by accident with his optic blast and asked Professor Xavier to replace him as deputy leader. Realizing Mimic would never take orders from another student, and needing his powers to face an upcoming mutant menace that Cerebro had discovered, Professor X accepted Cal's application and named him the new deputy leader. [X-Men (1st series) #27]

Despite aiding the X-Men in their struggles against Factor Three's minions, Mimic never managed to truly become one of the team. He was prepared to leave the team when they were assaulted by AIM's Super-Adaptoid, a power-duplicating android seeking to turn the X-Men into replicants like himself. Cal agreed to undergo the transformation process, hoping to gain new powers for himself. However, Mimic quickly realized that the Adaptoid was only interested in making lesser versions of himself as slaves, and so he fought back against the android. When Cal telepathically tricked the Super-Adaptoid into trying to mimic his own powers, the resulting feedback canceled out both their power sets. Though he had defeated the Super-Adaptoid, Mimic had lost his powers once more, though this time he lost them as a hero. [X-Men (1st series) #28-29]

Going into retirement, Calvin began dating Vera Cantor after some time, his former arrogance having cooled somewhat. Cal's powers started to intermittently return and disappear, growing stronger and more uncontrollable with each cycle. He soon began to actually leech power from people nearby, taking away their strength instead of merely duplicating it. Several people actually died as his power flared up around them, and so he exiled himself to the Canadian wilderness in hopes of protecting humanity. He then sent Vera out to collect their old friend Hank McCoy for his expertise in genetics, hoping that he could cure Cal somehow. The Beast began working on a means of helping Mimic, using machines transferred from Ronald Rankin's old labs, but his actions were stopped by the Hulk, who had been weakened by Mimic's powers and tracked the source of his weakness to Cal's cabin. Beast tried to hold off the Hulk, but Mimic realized it would never be enough. Recognizing the threat he posed to all life on Earth, Calvin flung himself onto the Hulk, absorbing the man-monster's gamma rays into his own body until he succumbed to radiation poisoning. [Incredible Hulk (2nd series) #161]

Hank and Vera left Rankin's body in the demolished lab but he wasn't as dead as he appeared to be. As his expanded power range began to diminish, Mimic's power locked onto Logan (then known as Weapon X) in the nearby Canadian wilderness. Mimicking Logan’s healing factor cleansed enough of the gamma radiation from Cal's body to save his life. His abilities continued to mutate and he not only copied Logan’s powers but was physically transformed into a perfect likeness of Logan including his adamantium claws. This latest facet of his transformation was likely due to his father’s own exposure to adamantium prior to his conception. Copying Logan's powers also gave Cal a form of savage insanity similar to Wolverine's own berserker rages. Fortunately, one of the many computers in Ronald Rankin's lab contained a facsimile of the late scientist's psyche as well as a pre-programmed holographic matrix. This sentient system somehow produced a supersonic beacon that helped to calm Rankin’s bestial urges.

Calvin lived for years in the Canadian wilderness as a wildman but eventually began to travel outside of the beacon's range. Not consciously aware of the beacon's effect, Mimic wandered down to the Carolinas and was soon overwhelmed by his mimicked savagery. He even killed some people in his blind rage. Realizing that he had lost control of himself, he began raiding pharmaceutical stores and ingesting massive amounts of tranquilizers in order to dull the constant pain and maintain a semblance of control. By sheer chance, the real Wolverine and the Hulk were in the area and Logan was suspected of being responsible for Cal's crimes. Their efforts to learn the truth behind Logan’s murderous doppelganger eventually led them to Mimic. Cal blamed the two of them for his condition and attacked them. Their conflict ended when Wolverine and the Hulk noticed a light show and a low-frequency signal that compelled them towards Dr. Rankin’s laboratory. Traveling to the cave, Mimic learned that his father’s devices had been calming him since his power mutated. Mimic decided that he did not wish to be dependent on drugs or technology to control himself and Wolverine offered another solution. Logan suggested he try focusing on finding inner peace using the Japanese traditions that had helped him. Calvin built a traditional Japanese style temple and seemed to find some measure of peace in the process. He managed to shed Logan's form and powers, returning to his original appearance. [Marvel Comics Presents (1st series) #54-61]