The Immortal has been around for many, many years. He is older than the legend of King Arthur, serving as one of his legendary knights. He discovered North America, and fought in the Revolution for the revolutionaries. He even served as President under the alias of Abraham Lincoln. Later, he became a superhero and a member of the original Guardians of the Globe.
The Immortal was seemingly killed by Omni-Man along with the rest of the Guardians of the Globe but living up to his name he survived (despite having been decapitated). He was revived by the Mauler twins, who, while reattaching his head, attempted to use a 'control beacon' to make him their slave. It did not work. The Immortal shortly thereafter survived being killed by Omni-Man a second time, this time being impaled through the abdomen and ripped in half. After the new version of the Guardians of the Globe failed to meet expectations, Immortal was brought in as a "consultant" for Robot, the team's leader, and when the team continued to underperform, Immortal was appointed the full-fledged leader in hopes of turning them around.
The Immortal was married to his teenage teammate Dupli-Kate until her apparent death in Invincible #40. He resigns from the team shortly after. It was later reveal that the Kate that died was another duplicate, and the Immortal began a relationship with the original Dupli-Kate.