When Mabus was eight year old, the Kazon overthrew the Trabe. He claimed to have known little about their oppression by his people, and to have gone from a happy child playing in a field to an orphan exiled in space from one moment to the next. His parents died in the uprising.
In 2372, Mabus and the rest of the Trabe Governing Council were among a group captured by the Kazon-Pommar, and Neelix met him in a Kazon prison. Mabus told Neelix that he had sent a signal to other Trabe vessels and that help was on its way. When Mabus' Trabe supporters liberated the prisoners, he rescued Neelix as well.
Mabus met with Captain Janeway and proposed an alliance. Mabus told her that he wanted peace with the Kazon and agreed to arrange a meeting with all the Kazon leaders to work out an agreement. He admitted that his people were wrong in enslaving the Kazon, and that this happened when he was a youth, and that most Trabe do not feel the same way that their parents did. Janeway convinced the many Kazon factions to meet with her and Mabus in order to restore order among the warring factions and provide safe passage for USS Voyager through their space.
Mabus betrayed Janeway however, and a Trabe ship attacked the conference in an attempt to kill the all of the Kazon leaders in one hit so the Trabe could take over once again. Voyager drove the Trabe ship off, Janeway informing Mabus that she would not countenance deals with murderers regardless of Voyager's vulnerable social position in the quadrant, but the damage had been done as the Kazon vowed vengeance on Voyager. (VOY: "Alliances")