Currently he is the lead designer for Adventure Time and if you follow Youtube cartoon channel Cartoon Hangover you will recognise his distinct style on Bravest Warriors. He blessed us with visual candy such as Impossibear (here) and cutness Catbug.
Not limited to animation he has his own comics; "The Babysitter"and "Escape from Dullsville" through SLG comics. In case you are wondering just how close Andy's storytelling is to Adventure Time, here's a synopsis of Escape from Dullsville:"Something born of extraordinary circumstances tends to allow for more incredible circumstances to happen," says creator Andy Ristaino of his immensely original comic book magnum opus, Escape from Dullsville, in which a fetus, tired of its surroundings, decides to take an early exit. Everyone is after the little fetus-lad - aliens, mad truckers, babysitters with jet packs, men in black, crazed monsters, masked wrestlers and, of course, the little fetus' mother (a crazed woman with a BIG knife)!" - Amazon
If that intrigues you, grab "Escape.." from Amazon right here