Mario was the #1 crime boss in the world and a major villain in Sonic For Hire. He is the main antagonist of Seasons 2 and 6.
After Sonic was told by Tails that he had to borrow money to bail him out of jail in "Ice Hockey", he ends working for Mario to work off his huge debt. In "Toad", Mario tells Sonic to kill Toad because he isn't doing his job which is watching the princess.
In "Angry Birds", he orders Sonic to kill "a group of pissed-off birds that have gotten a little to popular lately".
In "Double Dragon", he asks Sonic to kill off the brothers from Double Dragon but he ends failing the job due to poor assistance from Eggman and Earthworm Jim.
In "Frogger", Mario sends Luigi along with Sonic to keep an eye on him so he doesn't mess up but Luigi has another one of his episodes and gets killed by a bus.
In "Donkey Kong Country", Mario get's Sonic to go get some TNT barrels from Donkey Kong Country because "I feel like blowing up some worlds". Sometime after Sonic and Tails leave, Frogger shows up at Mario's office and tells him what happened.
Soon he pursues Sonic to Donkey Kong Country and is ready to kill him but he gets knocked into a cliff by Donkey Kong. Mario survives the fall however and calls up most of his and Sonic's enemies to kill him in "The Battle Part 01".
In "The Battle Part 02", most of Mario's allies get killed and he ends up dead after Sonic knocks him in a pit of spikes. His last words were "If you see the princess, tell her....tell her I said....she's a b*tch!". Due to his death, Sonic is made the new crime boss.
Mario appeared as a ghost in "The War Part 02", but he seemed friendly. His corpse appeared in "It's On", looking all rotten. In Season 4, the same corpse was swallowed by Kirby in "Movie Magic" so he could imitate Mario. He also made a small cameo in "Shadow the Hedgehog", where Tails was talking about using the Chaos Emeralds to revive Mario so they could build a new mafia and take over that one. Mario wasn't seen again until the Season 6 episode "Thunderhead", where he was revived thanks to time travel.
In "The Avengers", he orders Boba Fett, Ghost Rider, and Dog the Bounty Hunter to bring Sonic to him alive. However, they prove to be inept to Mario and when they come to encounter, Sonic fools them into thinking Eggman is Sonic, resulting the three to beat up the fat doctor. Earthworm Jim stops them and wonders what's going on. Mario then calls the Avengers to do the job for him. The Avengers capture Sonic because Sonic reached the Avengers later, thinking that they would help him against Mario.
Mario gets to the point and kills Sonic in "Return of the Plumber" by shooting him in the head. The death of Sonic makes world peace for everyone, giving Mario the opportunity to bring back his crime pals that died in "The Battle: Part 2" (with exceptions like Dr. Wily and Bowser) and started wrecking havoc. Unknown to him, Soniqua is alive and wants to hunt Mario down for what he did to Sonic.