McNichol was born July 2, 1961 in Los Angeles, California. He began his career at the age of 7, acting in a Band-Aid commercial. He appeared in roughly 80 commercials between 1967–1973, including spots for Kool-Aid and Crest. He landed minor roles in shows such as S.W.A.T. and Little House on the Prairie.

He made his first movie, Sunshine, in 1973 at the age of 12. His first regular starring role was as younger brother Jack on 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, produced by Phil & Mitch Margo.

When California Fever ended, he won the role as host of a weekly talk show, Hollywood Teen. He appeared in several successful made-for-TV movies, including Blinded by the Light which co-starred his sister Kristy. He made several low budget feature films before accepting the role of Josh Clayton on General Hospital). He performed in a band throughout the 1980s under the name "Jimmy James". His last notable acting role was that of Jill Ireland (played by Jill Clayburgh)'s son, Valentine McCallum, in the 1991 movie, Reason for Living.

He and his wife Renee (whom he married in 1997) have two children and live in Durango, Colorado.

McNichol is an avid Environmentalist, and in 1998 was focusing on ecological education with a web site called ECOTV.

In 2010 McNichol found out that he had a daughter, Kellee Maize a rap Artist, Songwriter, and Entrepreneur currently based in Pittsburgh, Pennsylvania.