A frequent collaborator with playwright-filmmaker David Mamet, Gregg's early film appearances included such Mamet efforts as "Things Change" and "The Spanish Prisoner." He later garnered substantial praise for his turn as a pre-operative transsexual opposite Adrian Grenier in "The Adventures of Sebastian Cole." Gregg made his debut as a feature film screenwriter with the paranormal mystery, "What Lies Beneath," turned in more respectable supporting work in films like "One Hour Photo," and appeared with regularity on such hit shows as "The West Wing." After writing and directing an adaptation of Chuck Palahniuk's novel "Choke," Gregg made his first appearance as secret agent Phil Coulson in the superhero spectacular, "Iron Man." More Coulson cameos followed, eventually leading to an expanded role in the summer blockbuster, "The Avengers," which teamed a number of Marvel Comics' most iconic heroes on screen for the first time. While not boasting the marquee recognition enjoyed by many of his costars, Gregg remained one of the more versatile and employable talents in Hollywood.

Born in Boston, MA on April 2, 1962, Gregg studied drama at New York University, where he was befriended and mentored by noted playwright, David Mamet. With Mamet's help, he and actor William H. Macy helped to co-found NYC's Atlantic Theater Company, where Gregg served as artistic director for a number of years. Over the course of one momentous year, he made his off-Broadway debut under the direction of Macy in Howard Korder's play, "A Boy's Life" in 1988, made his feature film debut in writer-director Mamet's mob fable "Things Change," and worked under Macy's direction again in the made-for-cable movie, "Lip Service." After making his Broadway debut in Aaron Sorkin's "A Few Good Men," and appearing on network TV for the first time in a 1991 episode of "Law & Order," he enjoyed a feature lead as a plumber mistaken for a podiatrist in writer-director Bashar Shbib's "Lana in Love," followed by another substantial role in the filmmaker's comedy "Ride Me."

Throughout the '90s, Gregg divided his time between performances on the stage and screen. Working on efforts in both New York and Los Angeles, he directed productions of Kevin Heelan's "Distant Fires" and Mamet's "Edmond," in addition to garnering praise for his performance in an off-Broadway mounting of Jez Butterworth's "Mojo." Gregg added further to his steadily growing film resume with solid turns in such notable films as "Clear and Present Danger," "The Usual Suspects," Mamet's "The Spanish Prisoner" and Paul Thomas Anderson's "Magnolia." Rounding out the decade, he made an indelible impression as the maternal, pre-operative transsexual step-father, Hank-Henrietta, in the indie coming-of-age drama "The Adventure of Sebastian Cole." As a writer, Gregg picked up his first screenplay credit for his work on the Robert Zemeckis-directed supernatural-thriller, "What Lies Beneath," starring Harrison Ford and Michelle Pfeiffer as a couple whose lakeside home hides a terrifying secret.

Gregg's ongoing collaboration with Mamet continued with a role in the writer-director's culture clash comedy "State and Main" and work in an off-Broadway revival of "Sexual Perversity in Chicago," also penned by Mamet. With the new millennium off to a promising start, Gregg later married "Dirty Dancing" actress Jennifer Grey in 2001. Continuing his string of supporting roles in wildly diverse projects, he played a robotics expert in director Steven Spielberg's homage to Stanley Kubrick, "A.I. Artificial Intelligence," then appeared alongside Mel Gibson in the Vietnam War drama, "We Were Soldiers," which Gibson also directed. In "Lovely & Amazing," the critically acclaimed ensemble comedy with Catherine Keener and Jake Gyllenhaal, Gregg played the husband of Michelle (Keener), a self-centered, struggling artist dealing with an unhappy marriage. That same year, he appeared in the dark psychological drama, "One Hour Photo," a well-regarded thriller starring an against-type creepy Robin Williams and directed by Mark Romanek.

Though Gregg's focus remained largely on the stage and film, he was also seen regularly on television with numerous guest spots on such popular shows as "Sex & the City," "The Shield" and Sorkin's "The West Wing" -- the latter of which provided Gregg with a recurring role as FBI Special Agent Michael Casper. He essayed real-life CNN news exec Eason Jordan in the award-winning docudrama "Live from Baghdad" then segued back to feature film, joining writer-director Mamet once again for the political thriller "Spartan," starring Val Kilmer. Gregg surfaced as a sycophantic corporate shark in the lauded comedy-drama "In Good Company," starring Dennis Quaid, Topher Grace and Scarlett Johansson and, amongst his many other projects, turned up in the family eco-adventure "Hoot," based on the young adult novel by Carl Hiaasen.

Gregg's return to screenwriting also marked his debut as a feature film director on the black comedy, "Choke." Based on the novel by Chuck Palahniuk, it starred Sam Rockwell as a recovering sex addict struggling to care for his Alzheimer's-afflicted mother (Angelica Huston). Gregg also appeared in the film, as did his father-in-law, actor Joel Grey. That same year, he entered what would become known as the Marvel Cinematic Universe for the first time when he picked up a small role as S.H.I.E.L.D. Agent Phil Coulson in the hugely successful comic book adaptation, "Iron Man" and its sequels, all of which starred Robert Downey, Jr. as billionaire inventor Tony Stark, the man inside the metal suit. He closed out the decade with such roles as Joseph Gordon-Levitt's boss in the indie romantic-comedy surprise hit "(500) Days of Summer," in addition to playing Julia Louis-Dreyfus' irresponsible ex-husband on all five seasons of the hit sitcom "The New Adventures of Old Christine."

After appearing as an uncaring penguin expert in the Jim Carrey family comedy, "Mr. Popper's Penguins," Gregg further expanded his Agent Coulson role in another epic superhero adventure, "Thor," featuring newcomer Chris Hemsworth as the titular God of Thunder. Having become a staple supporting character in the MCU, Gregg also voiced the character for the animated series, "Ultimate Spider-Man." Of course, it all led up to the hugely anticipated summer blockbuster, "The Avengers," in which Coulson struggled to help S.H.I.E.L.D. Director Nick Fury (Samuel L. Jackson) maintain order amidst a volatile team of superheroes consisting of Iron Man (Downey, Jr.), Thor (Hemsworth), Captain America (Chris Evans) and the Hulk (Mark Ruffalo).

Gregg will be reprising his role as Agent Coulson on the new ABC television series "Agents of S.H.I.E.L.D." in 2013.