Cain Marko’s mother died when he was very young, leaving him to live a life of psychological and physical torment at the hands of his abusive father, atomic researcher Doctor Kurt Marko. Following the death of his colleague Doctor Brian Xavier, Kurt married Xavier's widow Sharon and he and Cain took up residence in the Xavier's Westchester mansion with Sharon and her young son, Charles.
Kurt seemingly preferred Charles to his own son, which consumed Cain with jealousy and he took to bullying his stepbrother. Cain's father continued to beat him, but Cain did not suffer the abuse alone. Inexperienced at the use of his emerging mutant telepathic powers, Charles shared the pain and inadvertently learned of Cain's jealousy towards him. This only ensured that Cain would forever hate Charles for his unwitting betrayal.
After an argument with his father, Cain accidentally caused a fire to engulf his father's home laboratory. Kurt saved Charles first, and then went back for Cain, reinforcing Cain's belief that his father loved Charles more than he. Kurt died of smoke inhalation, but not before warning Charles to always beware of Cain.
Soon after, both Cain and Charles were drafted into military service and served in same unit. When Cain deserted under fire during a mission in Asia, Charles pursued, hoping to convince his stepbrother to return of his own accord. Charles followed Cain into a cave that housed the lost temple of Cyttorak, a powerful mystical entity. Therein, Cain unearthed the Crimson Gem of Cyttorak and, upon touching it, was transformed by its mystical energies into a human juggernaut. A subsequent cave-in buried Cain, and Charles believed his stepbrother dead.
Cain eventually dug himself free and made his way to America where he sought to use his newfound strength to exact revenge on Charles. However, he was defeated on every attempt by the X-Men, a team of mutant adventurers fighting to preserve Charles’ dream of peaceful coexistence between humans and mutants. Cain then formed a criminal partnership with the mutant mercenary Black Tom Cassidy, and the pair soon became two of the world’s most wanted criminals.
After many years of villainy, Cain was teleported to an alternate universe where he helped form the team of costumed adventurers known as the eXiles. Cain was eventually returned to his own universe, whereupon he encountered the psionic being known as Onslaught who used his enormous powers to physically hurl Cain from Canada to New York. There, he encountered the X-Men once more and was taken to Charles’ mansion for medical treatment. On waking, Cain sought out Charles to warn him of Onslaught. To his horror, Cain discovered that Onslaught was Charles who tore the Ruby of Cyttorak from Cain’s chest and trapped him within it. Therein, Cain discovered an entire plane of existence and encountered a corrupt aspect of Cyttorak. The mystic named Gomurr the Ancient sought to help free Cain from the jewel, imbuing him with the gem's power. Using his newfound strength, Cain destroyed the aspect and was freed from the gem, returning with more power than ever before. Ignoring Gomurr's warning that he was fated to destroy everything and everyone in his path, Cain returned to his villainous ways.
Soon after, Cain heard a rumor of a second Cyttorak Gem, and, coveting its power, he returned to the Temple where he found the first. However, it was a trap set by a Cult who sought the power of the Ruby for themselves, draining Cain's power to energize their Ruby. With the assistance of Black Tom and the X-Men, Cain claimed the power of the second Ruby, but it was possessed by an evil spirit which took over his body. The spirit made Cain smash through dimensional walls in an attempt to destroy its nemesis, but the natives of the other dimension ultimately freed Cain from its possession.
Cain eventually discovered that his becoming the Juggernaut was no accident. He had been compelled to enter the temple of Cyttorak, who was one of a pantheon of eight gods that each had an avatar on Earth. These avatars, named the Exemplars, enlisted Cain’s aid in constructing a machine that would remove humanity's free will, thus allowing their gods to rule the Earth. With Charles’ help, Cain regained control of his psyche and foiled the Exemplars’ plan.
Cain was again cast as a hero when the sentient spaceship Prosh returned to Earth to gather a group of costumed adventurers for a journey through time to uncover the keys to preserving human evolution. Whilst in the past, Cain was shown that despite all his yearning for power, he had squandered his mystical abilities. When the team reassembled in the present, they fought and defeated the alien entity named the Stranger who sought to control the natural evolution of mankind.
Soon after, Cain was drafted into the service of the Commission for Superhuman Activities as a super-powered bounty hunter. However, this new career was short-lived, and Cain eventually found himself working alongside Black Tom once more. Soon after, Cain requested the X-Men's help in stopping Tom, whose plant-based powers had resurfaced and flared out of control. After a brief clash with the X-Men, it came to light that Cain had been stripped of Cyttorak's mystical energies and was no longer invulnerable. Tom was defeated, but not before he knocked Cain into the surrounding waters. A young aquatic mutant named Samuel Pare saved Cain's life, forming a strong bond of friendship between the unlikely pair. When Cain was invited by his stepbrother to take up residence at the mansion, he accepted and soon came to appreciate his newfound lot in life. After assisting the X-Men on a mission, Cain joined the team.
After Sammy was returned home to Canada, Cain defied a restraining order to visit the boy. When he learned that Sammy’s father was physically abusing him, he flew into a rage and accidentally injured Sammy’s mother. Alpha Flight arrived to defuse the situation and, on seeing a terrified young Samuel, Cain surrendered himself and was taken into custody. Incarcerated in a holding facility designed to contain superhuman criminals, Cain was appointed a lawyer in the form of Jennifer Walters, the She-Hulk. Whilst Charles discussed a plan of legal action with Walters, another of the facility's inmates - the Rhino - broke free, setting Cain loose in the process. Rather than escape, Cain stayed and defeated the Rhino in order to save the life of a guard. This heroic act, combined with the Canadian government's less than ideal incarceration methods, was enough to make a case for Cain and endear him to Walters.
Soon after, Cain and Walters began a romantic relationship that was cut short by the arrival of Cyttorak's new avatar. With Walters’ aid, Cain defeated the new Juggernaut, but the damage he had caused during the battle forced Walters to rethink her feelings for him. An impassioned plea from Sammy’s mother led to Cain receiving a reduced sentence, as well as extradition back to the United States. Cain continued to serve as a member of the X-Men whilst undertaking community service and anger management therapy.
After the recent decimation of the mutant population following House of M|"M-Day," Cain journeyed to England where he joined a new incarnation of the British super-team Excalibur. Following an incident involving the Captain Universe entity, Juggernaut was left in a weakened state and has been assigned to the Thunderbolts program under the supervision of Luke Cage.