Official Canon Edit
Mortarion was found on Barbarus, a planet covered in toxic fumes. On the highest peaks where the air was most poisonous lived the Overlord, who claimed Mortarion as his foster son. The Overlord kept him caged to acclimate to the poison, and taught him the art of war. Mortarion would later live among the farmers in the valleys, and taught them warfare too after he fended off an enemy warlord with a scythe.
Eventually the only warlord left was the Overlord himself. At this time the Emperor appeared, and challenged Mortarion to slay the Overlord alone, or swear fealty to him. Mortarion was too weak to reach the top of the peak where the Overlord lived, so the Emperor finished him off himself, earning Mortarion's undying hatred. Mortarion was given the Death Guard legion, who specialized in bio-weapons and rad-weapons, as they could endure far more harsh conditions than their brothers. He also really hated psykers, taking a leading role in the anti-psyker faction at the Council of Nikea.
Horus Heresy Edit
Mortarion didn't need much convincing to join Horus, having come to hate both his father and the civilian leadership of the Imperium, wanting the strong to rule over the weak. Shortly after the burning of Prospero, he unsuccessfully tried to convince Jaghatai Khan to join them. When that failed and the Khan drove him off, Mortarion picked up a daemon of Nurgle to learn how to fight Daemons. Despite his hatred for psykers, Mortarion began using psychic powers himself, and quickly became a paranoid lunatic. His first captain Typhon killed all of his navigators claiming they were loyalists, and convinced Mortarion he could travel the warp without them. This proved to be a trap however, as the Death Guard were now in the plaguefather's grip. The only way to save themselves was to swear allegiance to Nurgle. While his corruption was now complete, Mortarion began to hate himself more than ever, as he was now everything that he despised.
Mortarion became a Daemon Prince after the Heresy, when he occupied the plague planet and made it more like Barbarus. Nurgle was apparently pleased by this act of cultivation. At some point though, Kaldor Draigo carved the name of the Grey Knight Geronitan onto his heart.
TTS Canon Edit
The Emperor claimed "Mortarion smelled like shit from day one."