From the Super Mario Wiki
Crimewave Clyde is a large, muscular, and aggressive prisoner from the Real World who had been sentenced to jail for 12,423 years with no parole due to his extensive criminal record.
In The Adventures of Super Mario Bros. 3 episode "Crimes R Us", Bowser and his Koopalings had traveled to Clyde's prison on the Doomship, where Bully Koopa abducted him. Under the request of Bowser, Clyde agreed to teach the Koopalings everything that he knew about crime, so that they could grow up to be hardcore criminals. Bowser also told Clyde that he could keep half of whatever was stolen in exchange, but he crossed his fingers behind his back when he said so.
Clyde's first lesson to the Koopalings was bank robbery. After they received a crash course from Clyde, the Koopalings raided the First Mushroom Bank, but were foiled by Mario and Luigi in the process, although they did manage to escape.
Realizing that the Koopalings failed lesson one, Clyde made some adjustments to his lesson plan, making sure that getting rid of the Mario Bros. was lesson two. The first step in this lesson was to have Cheatsy steal the lunch money from a Mushroom boy in Desert Land, whose cry for help traveled through a Warp Pipe and alerted the Mario Bros. The boy directed Mario and Luigi to a pipe, which trapped them in an underground maze where Clyde and Big Mouth locked them in with a Boom Boom.
Afterwards, claiming that he had taught the Koopalings everything he knew about crime, Clyde let them run amuck in the Mushroom Kingdom, where they stole whatever they chose to steal. However, Clyde was pushed too far when the Koopalings not only started stealing from each other, but his own pockets as well.
When Bowser proposed to have his Koopalings steal one billion Gold Coins from the Mushroom Kingdom Treasury, Clyde only agreed to help, just because half of the loot would be enough to relieve him from all the stress the Koopas were causing him. To Clyde's surprise, the Koopalings double-crossed him, and handcuffed a Bob-omb to his ankle, although he managed to extinguish the fuse before it blew up. Fed up with the Koopalings, Clyde knew he needed help.
Clyde broke into the maze that Mario and Luigi were trapped in, and begged them to get him back to the Real World. After offering to help them stop the Koopalings from robbing the Mushroom Kingdom Treasury, the Mario Bros. reluctantly agreed.
After the Koopalings invaded the Mushroom Kingdom Treasury and began packing up the loot, they were surprised to see Clyde with Mario and Luigi waiting for them behind a vaulted door. Before the Koopalings could react, Clyde and the Marios tied them up, and sent them up into the Doomship, along with a Bob-omb. When the Doomship had been partially blown away, fireworks were formed, while Clyde felt proud of the fireworks.
However, the Mushroom Kingdom is not recognized by the United States government, which means that Clyde's heroism there could not be considered a case for clemency. As Clyde was still a prisoner in the Real World, Mario and Luigi were obliged to return him to his jail cell, which Clyde agrees to. Following his return to prison, he was then befriended by Mario and Luigi, who would pay him visits at the prison's visitor center. While the Mario Brothers consider Clyde's sentence onerous especially since he has reformed, Clyde notes that he will gladly serve his 12,423-year prison sentence with no parole compared to spending one more day with the Koopas.