Additional theatre credits include Jonathan in the Alan Menken/Tim Rice musical King David, Harlequin in Triumph of Love, Snoopy in the Broadway revival of You're A Good Man, Charlie Brown (for which he won the Drama Desk Award and a Tony), Carmen Ghia and later Leo Bloom in The Producers (earning Drama Desk and Tony nominations), and The Frogs at Lincoln Center, which reunited him with fellow Producers star Nathan Lane and Susan Stroman.

In 1996 and 1997, Bart appeared as Bud Frump in the USA national tour of How to Succeed in Business Without Really Trying.On television, Bart played George Carlin's son on The George Carlin Show (1994), and on Bram and Alice (2002) he portrayed Bram's assistant Paul Newman. He became widely known to viewers with his portrayal of George Williams, the murderous pharmacist in love with Bree Van de Kamp (Marcia Cross), on Desperate Housewives, which earned him a SAG Award.

Bart provided the singing voice for Hercules in Disney's Hercules, as well as the singing voice of Scamp in Lady and the Tramp II: Scamp's Adventure. He was featured in the 2004 remake of The Stepford Wives with Nicole Kidman, Matthew Broderick, Bette Midler, Christopher Walken and Glenn Close, and in The Producers (2005), in which he reprised his role of "common-law assistant" Carmen Ghia.

In December 2006, Bart played Howard "The Weasel" Montague in the Sci Fi Channel miniseries The Lost Room. In June 2007, he starred as Stuart in Hostel: Part II, the sequel to 2006's Hostel. In 2007 He had supporting roles in American Gangster, and Spy School. In 2008, he appeared in Harold & Kumar Escape from Guantanamo Bay, the sequel to 2004's Harold & Kumar Go to White Castle, and Midnight Meat Train.

Bart also originated the lead role of Dr. Frederick Frankenstein in the musical adaptation of Mel Brooks' Young Frankenstein, which opened on Broadway in November 2007, following a run in Seattle. He recently reprised the role alongside former Broadway co-stars Shuler Hensley and Cory English in the United States national tour which launched in September 2009.He ended his run on August 8, 2010 and was succeeded by Christopher Ryan. Bart originally portrayed the role of Igor in the original workshop read through.

Brad Oscar and Bart reprised their roles as Max Bialystock and Leo Bloom, respectively, in a production of The Producers at Starlight Theatre in Kansas City, Missouri. The show ran from August 23-29, 2010.

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