After interim guitarist Donnie Dacus left Chicago in late 1979, Pinnick was hired as a session musician for the album Chicago XIV. He continued to tour with the band, and remained as a second guitarist despite the hiring of Bill Champlin for Chicago 16. By the time Chicago 17 was released in 1984, Pinnick was listed as a full member of the band. However, Pinnick himself is quoted as saying he was never a member and in any case he left the group in 1986 and was succeeded by Dawayne Bailey.
Pinnick's other recording credits include work for Peter Cetera, Chuck Negron, Chet McCracken and Rick Devin. As of 2008, he continues to do session work in Los Angeles, where he is also the co-owner and operator of a recording studio.