He started his career as a singer-songwriter in the mid-1970s, playing in such Greenwich Village coffee houses as Paul Colby's The Other End.
He honed his playwriting skills as a regular contributor to The West Bank Downstairs Theater Bar repertory during the 80s, then went on to create several genre-bending musical theater pieces, including his music-drama about folk singers during the blacklist Three Songs (Fremont Centre Theatre, 1997, revived in 2002), garnering "Critic's Choice" in the Los Angeles Times and a "Best Ensemble" Nomination (LA Weekly Theater Awards).
In 2000 he released an album, Campaign Songs, as an accompaniment to his drama Muckrakers: an evening of presidential campaign songs and family dysfunction, which debuted at FCT on the eve of the United States presidential election.
A graduate of the Yale School of Drama, Mesnik's theatrical resume encompasses Broadway (La Bête; Oh! Calcutta!), Off-Broadway (Modigliani; A Weekend Near Madison; Smoke On The Mountain; The Good Times Are Killing Me; The Rimers of Eldritch - and others), major regional venues such as Yale Rep, The Old Globe, McCarter Theatre, The Kennedy Center, and Actors Theater of Louisville, and European-American collaborative productions of Shakespeare's King Lear and Chekhov’s Ivanov (Moscow Art Theatre). In 2002, he was nominated for an Ovation Award for his role as Holofernes in Shakespeare's Love's Labours Lost,produced by A Noise Within.
He became a familiar face in the 90s and 2000s from his numerous commercial, episodic television and film appearances, including: L.A. Law, Law & Order, Lois and Clark, Murphy Brown, That '70s Show, Spin City, 3rd Rock from the Sun, Dharma & Greg, Curb Your Enthusiasm, Minority Report, Titanic, Stonebrooke, and two films by John Schlesinger: The Next Best Thing and Eye for an Eye.