Zach Grenier's career began in the theatre. As a young actor, he toured with Boston-based little Flags Theatre, acting and playing guitar in towns throughout the country in two shows, The Furies of Mother Jones and Winds of People. Subsequently, he appeared as Jacques in a Boston Shakespeare Production of As You Like It, and months later, played the leading role of the Captain at Portland Stage in Play Strindberg. In 1978, he moved to New York City to pursue a stage career. It was a challenging time, but he was fortunate enough to obtain the all-important Actors' Equity card by performing with what was then called Performing Arts Repertory Company, or PART, and what is now known as Theatreworks. He toured with the musical Teddy Roosevelt, playing Roosevelt's rival New York Senator Chester Conklin. He made an important theatrical connection by studying with Paul Austin, which led to him joining Ensemble Studio Theatre.

Ensemble Studio Theatre (EST) became his artistic home, and he performed there in three of the EST marathons in Water Music by Michael Erickson, Cross Patch by David Mamet, and Death and the Maiden by Susan Kim, as well as participating in numerous readings and workshops. With EST providing a base, Zach pursued theatre work in a variety of venues. In this period, he performed at Hartford Stage in The Tooth of Crime, the Alley Theatre in Hunting Cockroaches, and toured in Citizen Tom Paine in a production that originated at the Williamstown Theatre Festival.

He also appeared in New York experimental theatre as Baal in the Brecht play by that name, in Richard Foreman's The Cure,  Jeffrey Jones' Der Inca von Peru and Tomorrowland, Kathy Acker's Birth of the Poet, directed by Richard Foreman, and Mac Wellman's Sincerity Forever. In 1988, he performed the on-stage character Sid Greenberg  as well as multiple voice roles in Eric Bogosian's Talk Radio at the Public Theatre. His chameleon tendencies next proved useful in the Broadway production of Larry Gelbart's  Mastergate, in which he played all four lawyers in the play. He worked with Julie Harris in Tim Mason's The Fiery Furnace at Circle Repertory Theatre and Dame Harriet Walter in Three Birds Alighting on a Field at the Manhattan Theatre Club. 

He originated the role of Dr. Chapman in David Rabe's A Question of Mercy at New York Theatre Workshop, a performance that earned him an Outstanding Artist honor from the Drama League. While he was performing in that play, he was cast in ABC's C16:FBI, and he was off to Los Angeles. Between television and film engagements, Zach takes every opportunity to return to stage work. His portrayal of Dick Cheney in David Hare's Stuff Happens at the New York Shakespeare Festival was recognized with an Ensemble Award by the Drama Desk and a Drama League nomination for the Ensemble.

It was not long after that inspiring experience that he found himself cast as Beethoven in Moises Kaufman's 33 Variations at La Jolla Playhouse. These theatrical experiences were the impetus for a move back to the NYC area. Shortly after his return, he found himself back at EST in Vern Thiessen's Lenin's Embalmers. Later that year, 33 Variations moved to Broadway, with Jane Fonda leading the cast. Zach's performance as Beethoven brought him a Tony Award nomination.

Since being back on the east coast, Zach has twice shared the Roundabout Theatre Company's Broadway stage with Frank Langella -- in Robert Bolt's A Man for All Seasons and Terrence Rattigan's Man and Boy. He has also worked three times at the Atlantic Theater Company - in Moira Buffini's Gabriel, John Patrick Shanley's Storefront Church, a performance that garnered him a nomination for a Lucille Lortel Award, and Rajiv Joseph's Describe the Night. During the 2015 hiatus of The Good Wife, he played Mephistopheles to Chris Noth's Faustus in Doctor Faustus at the Classic Stage Company.

A major career highlight was the role of Willy Loman in Death of a Salesman at the Pittsburgh Public Theater.

Zach continues to regularly employ his theatrical talents by working for Bryan Doerries· Outside the Wire group, giving readings of Ancient Greek plays for military service people and other survivors. In 2018, he appeared in Antigone in Ferguson, a Theatre of War production under the aegis of the Harlem Stage.