Westenberg is best known for his work in Broadway musicals, appearing in the original casts of Sondheim's Sunday in the Park with George (as Alex and the soldier and replacing Mandy Patinkin as George) and Into the Woods (originating the Wolf and Cinderella's Prince), and reprising his roles in the TV versions. He also played Javert in Les Misérables and Dr. Lyman Hall in the revival of 1776, among others. Film credits include The Ice Storm and Before and After, while on television, he recurred on One Life to Live and has appeared on Third Watch and Law & Order: Special Victims Unit.
Now living in Missouri, Westenberg teaches acting at Drury University in Springfield and performs periodically at the Muny Theater in St. Louis, playing Captain Hook and Mr. Darling in Peter Pan and Mr. Brownlow in Charles Dickens' Oliver!