Clark began his acting career as a founding member and former artistic director of the Atlantic Theater Company in New York. He has acted in numerous productions with the company including Boys' Life at Lincoln Center, Mojo, The Night Heron, Sexual Perversity in Chicago, and most recently Happy Hour, written by Ethan Coen. Other New York stage credits include Aaron Sorkin's A Few Good Men on Broadway, Unidentified Human Remains, and A. R. Gurney's, The Old Boy.
Gregg moved to Los Angeles in the mid-nineties to pursue film and television work landing roles in films including Mr. Popper's Penguins, Thor, Iron Man 2, 500 Days of Summer, Choke, Iron Man, In Good Company, Spartan, State and Main, Lovely and Amazing, The Human Stain, We Were Soldiers, One Hour Photo, and Magnolia. He played the role of Hank/Henrietta in Tod Williams' debut feature The Adventures of Sebastian Cole for which he received an Independent Spirit Award nomination. In 2012, he reprised his role as S.H.I.E.L.D. Agent Phil Coulson in Marvel's The Avengers, which broke many box office records including the biggest domestic opening weekend of all time.
This year, Gregg will be seen in several new films including Joss Whedon's Much Ado About Nothing, CBS Films' The To Do List, Very Good Girls with Dakota Fanning, and Jason Reitman's Labor Day, alongside Kate Winslet and Josh Brolin.
On television, Gregg is known for his recurring roles on The West Wing, Sports Night, Will and Grace, Sex and the City and The Shield, among others and for co-starring with Julia Louis Dreyfus on the long-running CBS comedy The New Adventures of Old Christine.
An experienced screenwriter and director as well, Gregg's screenwriting debut, What Lies Beneath (Dreamworks) starred Harrison Ford and Michelle Pfeiffer and was directed by Robert Zemeckis. His feature film directing debut, Choke, which he adapted from the novel by Chuck Palahniuk, starring Sam Rockwell and Anjelica Huston premiered at the 2008 Sundance Film Festival, was awarded a Special Jury Prize for Best Ensemble, and was released by Fox Searchlight in 2008. Clark has written or re-written scripts for Universal, Disney, Paramount, Warner Brothers and Fox 2000.
Gregg's second film as writer-director, and this time, star, the dark indie comedy, Trust Me, premiered at the 2013 Tribeca Film Festival. In the film, which also stars Sam Rockwell, Allison Janney, Felicity Huffman, William H. Macy, and Amanda Peet, Gregg portrays a loser agent for child actors and former child star, Howard Holloway whose life is turned upside down when he suddenly finds himself representing the hottest young star in town.
Also a theater director, Gregg directed Kevin Heelan's Distant Fires, which was nominated for Drama Desk and Outer Critics Circle awards in NY and moved to the Circle-in-the-Square downtown for an extended run. The Los Angeles production, starring Samuel L. Jackson, won three L.A. Weekly awards including Best Direction, Best Ensemble and Best Play and was nominated for four Ovation Awards including Best Director. He also directed the acclaimed 1998 Atlantic revival of David Mamet's Edmond, and created, co-wrote and directed the Los Angeles serialized play, The Big Empty.