He had his first film appearance with an uncredited role in "Sunshine" (1973) at the age of 12. His first regular starring role was as younger brother Jack on the network series "The Fitzpatricks". Michele Tobin played his sister, Mo, on the show, and they later worked together on "California Fever". McNichol sang the show's theme song and in 1978 recorded an album with his sister, "Kristy and Jimmy McNichol", produced by Phil & Mitch Margo. The album spawned one hit single, a cover of The Chiffons' "He's So Fine", which charted at #70 on the Billboard Hot 100 in August 1978. The siblings also appeared as co-hosts of the fall 1978 "ABC All-Star Saturday Preview Special", a youth-oriented sketch comedy show featuring musical guests such as the Bee Gees and Donny Osmond. McNichol and his sister, however, ceased performing together after Kristy had a manic breakdown while the two were in France; she was diagnosed with bipolar disorder. Afterward, McNichol stepped away from the music industry, later commenting: "I know the outcomeit's a real big high one year, and the next year, nobody knows who you are. All that singing and touring and the guys behind you doing drugs. Eventually it's gonna get to you."