From the Tommy liner notes by Brian Cady:
According to John, Pete commissioned him to write this song and "Fiddle About" during a discussion in Detroit during the "Magic Bus" tour, which would make it July 13, 1968. John was sent off to "write something horrible."
"I actually based Cousin Kevin on a boy who lived across the street. Our parents thought we should play together, and he was completely sadistic. Musically, I wanted the piece to sound both sinister and childish, so I came up with a kind of ‘Chopsticks’ theme on the piano to make it sound like a simple children’s song."
In an early song list, this song followed "Fiddle About" and was preceded by the bridge song "Cousin Kevin Model Child" which was finally released on the 1998 Odds & Sods CD. It was then followed by a song called "Dream (School Song)."
"Cousin Kevin" was rarely performed in the live version.