Member of the Koopa Troop
Considering how easily he usually stomps on all their faces, Mario would be a shoe-in for Bowser's illustrious army of hooligans and wizards. He'd infiltrate the Koopa Troop from the bottom and quickly work his way up the ranks to confront Bowser and challenge him for his spot as the head honcho, all while on the terrible turtle's payroll.
Before you say it, we know—Mario joined the Koopa Troop in Super Mario RPG on the SNES. But that's just what Bowser told himself in his attempt to keep up appearances as he joined Mario's party to combat the giant, talking sword that had recently crashed into his castle. Obviously.