Baldwin was born in Massapequa, New York, the son of Carol Newcomb (née Martineau), a breast cancer survivor who founded the Carol M. Baldwin Breast Care Center of the University Hospital and Medical Center at Stony Brook, and Alexander Rae Baldwin, Jr., a high school history/social studies teacher and football coach. Baldwin was raised in a Catholic family of Irish and French descent. In addition to his three famous actor brothers, Alec (born 1958), William (born 1963) and Stephen (born 1966), Baldwin has two sisters, Beth Baldwin Keuchler (born 1955), and Jane Baldwin Sasso (born 1965).

Baldwin may be best known for his role as detective Beau Felton on the television series Homicide: Life on the Street. Baldwin as Beau Felton on Homicide: Life on the StreetHis acting debut was in the TV movie "Too Good to Be True" in 1988. He followed this with feature roles in the films Born on the Fourth of July (1989), Harley Davidson and the Marlboro Man (1991), and Hero (1992). He returned to television in 1990 for the short-lived sitcom Sydney, which starred Valerie Bertinelli.

In 1993 he was cast in the Baltimore-based cop show Homicide: Life on the Street. Although he received good reviews he left in 1995 after three seasons, and his character was later killed off. He returned to TV movies such as Attack of the 50 Ft. Woman, Family of Cops, and Twisted Desire, as well as the 1996 feature films Mulholland Falls and Trees Lounge.

In 1998 he was set to film the romantic comedy It Had to Be You, but was replaced by Michael Rispoli after being arrested for cocaine possession. John Carpenter's Vampires was released during his subsequent stint in rehab.

Baldwin returned to appearances in various TV series episodes and TV and direct-to-video movies, including reprising his role as Beau Felton in Homicide: The Movie in 2000.

More recently, Baldwin has starred in TV movies such as Anonymous Rex (2004) and Our Fathers (2005) and small-budget films including Sidekick (2005) and Moola (2006). In 2005, he was featured in VH1's Celebrity Fit Club, a reality show in which overweight celebrities compete to see who can shed the most pounds. He was the team captain of the Eastsiders, which also featured rapper Biz Markie, plus-size model Mia Tyler, and actor Joe Gannascoli (later traded for Judge Mablean Ephriam from Divorce Court).

In 2008, Baldwin appeared on VH1's reality series Celebrity Rehab with Dr. Drew and will be seen as Julius Krug in the upcoming HBO film based on the 1975 documentary Grey Gardens.