Judd Nelson was born in Portland, Maine, the son of Merle, a court mediator and former member of the Maine state legislature, and Leonard Nelson, a corporate lawyer who was the first Jewish president of the Portland Symphony Orchestra. He has two sisters, Eve and Julie. He went to school at St. Paul's School in Concord, New Hampshire, and studied at Haverford College in Pennsylvania, leaving part way through his sophomore year. He subsequently moved to Manhattan where he studied with Stella Adler.

Nelson was one of the 1980s "Brat Pack", starring in such films as Joel Schumacher's St. Elmo's Fire and John Hughes' The Breakfast Club (which was his most popular role as rebel "John Bender"), as well as providing the voice of Hot Rod/Rodimus Prime in Transformers: The Movie. He also starred in a film titled Making the Grade and co-starred alongside a young Kevin Costnerin the road-rip movie Fandango (1985 film).

Following his starring role in The Breakfast Club, Nelson's career failed to take off in a big way, as several of his films, including Blue City (co-starring Breakfast Club alumna Ally Sheedy) and From the Hip, performed poorly at the box office, and he played a serial killer in Relentless. His career lost momentum for a time, and in the 1990s he appeared in urban-themed dramas such as New Jack City and Light It Up. However, in 1996 he received a starring role on the NBC television sitcom Suddenly Susan, which saw enormous success for a four-season run. Nelson made a cameo appearance in the 2001 film, Jay and Silent Bob Strike Back; the film's writer and director, Kevin Smith had been a long time fan of Nelson and the "Brat Pack" movies. Throughout the late 1990s and 2000s, Nelson mostly appeared in television roles and lower-budget films, although he also acted on stage.

Nelson also recently appeared in the television show CSI: Crime Scene Investigation as a friend of a murder victim. He plays the host in The Real Fear Factor, a spoof on Fear Factor, in National Lampoon's TV: The Movie.