Bonsall was born in Torrance, California. At the age of five, he won the role of Andy Keaton on the sitcom Family Ties. Bonsall won three Young Artist Awards for his performance on the series. He was also nominated for a Young Artist Award for his starring role in the made-for-TV movie Do You Know the Muffin Man? in 1990.
He made his feature film debut in 1992 in the horror film Mikey, playing the title role, a demonic young boy who murders his parents and terrorizes his adoptive parents. This caused controversy in the UK owing to its depiction of a child killer at a time when the murder of James Bulger had raised sensitivity about violent behavior by children. The film was initially passed uncut by the British Board of Film Classification, but its certificate was withdrawn by James Ferman in the wake of the Bulger killing.
In science fiction circles, he is known for the part of Alexander Rozhenko, son of Star Trek: The Next Generation's Klingon security officer, Mr. Worf—a recurring role he played for 7 episodes across several seasons.
He played Patrick Swayze's son in the 1993 film Father Hood. In 1994, he starred in the Disney comedy Blank Check and co-starred with Bob Saget in the TV movie comedy Father and Scout. His other acting credits include the TV movie Mother Goose Rock 'n' Rhyme and guest appearances on television series such as The Super Mario Bros. Super Show! and The Young Riders.
In 1995, Bonsall retired from acting and moved with his mother and stepfather to Boulder, Colorado. He attended Boulder High School, graduating in 2000. He also became a musician, forming the rock band Late Bloomers with his friends in 1998. He has also been in the Boulder-based punk bands Thruster as well as The Light on Adam's Stereo. He was in the Los Angeles-based punk band This Life of Mine, but is also working on solo projects.
In a 2005 interview, he stated that he has no interest in returning to acting.
On March 28, 2007, Bonsall was arrested on charges of assaulting his girlfriend. He was sentenced to 2 years of probation on August 31, 2007.
On December 7, 2009, Bonsall was arrested for third-degree assault and failure-to-appear in connection with the 2007 assault on his girlfriend.
In February, 2010, Bonsall was arrested on charges of using marijuana in violation of the terms of his release. He was sentenced in April 2010 to 2 years of probation.
As of 2015, Bonsall plays guitar and sings in two bands, Lowjob and Bootjack & Bonz.
As of 2016, Bonsall has been touring with rock band The Ataris.