As a child, Largo was accepted to the Juilliard School of Music, but never got the letter as it was frozen atop Mt. Springfield. His assumed rejection from this school could explain his eventual lack of passion for music.
Mr. Largo is the music teacher at Springfield Elementary School. He is one of the more uptight and mean teachers at the school. Largo teaches Lisa's band class where he often stifles her creativity. He only teaches simple, boring and unoriginal songs to the band causing Lisa to often cause outbursts in class, which makes him gonna mad and sometimes makes he wants to kill Lisa. Despite this, he can occasionally encourage excellent performances from his students, leading them in playing "Living in America."
Once, had to take over for Groundskeeper Willie for a short time while Willie was working at The Gilded Truffle. He turned out to be even worse at being groundskeeper than music teacher. He once carpooled with Waylon Smithers, but ended up getting into a multi-car pileup thanks in part to Bart stealing the sign "Bart Bvd." as part of a dare by Jimbo Jones, Dolph Starbeam and Kearney Zzyzwicz. When Homer (dressed as the Safety Salamander) saved the two of them from the car, Largo stated that they were carpooling, and ended it right there. He wanted to have a sex change operation to look like Julie Newmar, but ended up becoming shorter instead because Dr. Nick mixed up his surgery file with Moe's. Dewey Largo presumably had Nelson fix up his bike, but then, due to Nelson unknowingly using a Water Soluble glue, it collapsed during a parade he was in (implied to be a gay pride parade), and he angrily returned it to Nelson. He graduated Springfield University.
In "Flaming Moe", Dewey quitted and claimed to have found his soulmate (also named Dewey) and went to Moe's. However, he is still seen in the opening sequence and later episodes, teaching the music class.
In "Girl's in the Band", he was shown waking up in the morning sharing the bed with a man who apparently stays home all day while Dewey works.