Frechette first appeared on the professional stage at the Edinburgh Fridge Festival as part of the Rhode Island Summer Ensemble, starring with Chel Chenier in the comedy Pontifications on Pigtails and Puberty in 1979.

He received high praise in 1981 for his work in two different productions of Harry Ruby's Songs My Mother Never Sang. The same year he starred in the One Act off-Broadway production of In Cahoots, part of the Three Hopefuls MARATHON (featuring two other one-act productions). He left to work in Los Angeles, but returned in 1984 to star in Bob Merrill's Musical We're Home, and again in 1987's revised production of Flora, The Red Menace (he also recorded songs for the cast album).

Frechette's made his film debut in Grease 2 (1982) as "T-Bird Louis DiMucci". He appeared on the film's Soundtrack (including a solo version of 'Let's Do It For Our Country', a satirical duet ballad he performed with Maureen Teefy in the film). He went on to appear in the pilot of The Voyagers (as Eddie Rickenbacker).

He had a large role in the 1984 comedy No Small Affair as Jon Cryer's older brother. He made guest appearances on Taxi, The Renegades, Hill Street Blues, Hotel, It's a Living, Cagney & Lacey, Matlock, and, most notably, in two episodes of L.A. Law as Christopher Appleton, an HIV-positive gay man who claimed to have killed his lover as an act of mercy because he was dying from AIDS.

Frechette's partner since 1988 is Director David Warren They married in 2017.

In 1991 he starred opposite Cherry Jones in Our Country's Good, which resulted in a second Tony nomination for best actor. The same year he co-starred in Absent Friends as grief-stricken Colin. In 1992, he appeared in Bob Merrill's last Broadway musical (and cast recording) of Hannah...1939 and Larry Kramer's autobiographical The Destiny of Me. He also appeared on Broadway in the original productions of Any Given Day (1993) and The Play's the Thing (1995), as well as the 2005 revival of The Odd Couple as Roy (and understudy for Matthew Broderick's Felix Unger).

The same year he appeared in the first season of Law & Order as Jack Curry in 'The Reaper's Helper", an HIV positive man committing "mercy killings" for others diagnosed with AIDS. He would appear in two more episodes of Law & Order (and one episode of Law & Order: Criminal Intent). In 1992, he had a recurring role in the first season of Picket Fences.

In 1993 he costarred in the critically acclaimed Barbarians at the Gate. The following year, he starred in the Lifetime film Against Her Will: The Carrie Buck Story. In 1996 he appeared in the pilot for the NBC series Profiler as computer expert/hacker George Fraley. Between shooting the pilot and being picked up, the producers made his character a regular on the show and would remain on the show throughout the entire run.

He's appeared at the Roundabout Theater in three productions; 1995's The Play's the Thing, 1999's Hurrah at Last, and 2002's The Dazzle. For The Dazzle, he and Reg Rogers were both nominated for Drama League Award nomination for Distinguished Performance and Lucille Lortel Award for Best Actor (Rogers won); the two shared the Obie Award for best actor for their performances as the co-dependent Collier Brothers. They originated the show in 2000 at the New York Stage and Film at Vassar College's Powerhouse Theatre.

Frechette was part of the west to east coast transfer of Hurrah at Last, in the role of Laurie. He earned the Backstage West Garland Awards in 1998 for his performance produced by the South Coast Repertory. He also appeared in their productions of Night and Her Stars and The Extra Man. In 2016 he returned to the theater company in the role of Joseph II in Amadeus.

In 2006, he took on the substantial role of Peter Hammond, the bank manager, in Spike Lee's heist film Inside Man. He also appeared in small roles in various films, including First Wives Club and Miracle at St. Anna. In 2016 he appeared as a high-powered divorce attorney on the television series Devious Maids.

Was a member of the resident ensemble at the Oregon Shakespeare Festival for five years (2011-15). He has also worked with national regional companies including the Seattle Repertory Theatre, Berkeley Repertory Theatre, George Street Playhouse, Cape Playhouse, The Old Globe, La Jolla Playhouse, Mark Taper Forum, Berkshire Playhouse, and the Pasadena Playhouse as an actor and instructor since leaving New York City.