Come to think of it, this is pretty strange.

Look at Mario's tools. Mushrooms, feathers, raccoon outfits — what do any of these have to do with gaskets or faucets? Come on, man, show me a wrench! A pipe-snake! Anything! Who's going to hire a plumber thatshows up at their house with a toolbox that has nothing in it but a tin whistle, a flower, and a strange-looking mushroom? That just screams lawsuit.

Blogger Jake Christie analyzes (and debunks?) the longstanding claim that Mario is a professional plumber, citing such oddities as his reliance on overalls and pipes to carry the image, and his story of origin in which he gets sucked down some tubes he should have known to turn off beforehand. Eliza "Series of Tubes" Gauger

Completely awesome Mario Bros illustration from NowGoCreate.

there is very little evidence of Mario's skill as a plumber [JakeChristie, thanks Nycjon]