Gregg was a founding member, and later artistic director, of the off-Broadway
Atlantic Theater Company, which formed in 1983. Gregg has been featured in a number of supporting roles in films, such as
Lovely & Amazing,
The Human Stain, and
In Good Company, and a number of guest spots on TV series, such as
Will & Grace,
Sports Night,
Sex and the City and
The West Wing. He wrote the screenplay for the 2000 thriller
What Lies Beneath. Gregg consulted his father, a retired religion professor at
Stanford, for the quotation from
Saint Paul's
letter to the Galatians which Gregg used in
Choke. Gregg's father is the former chaplain at
Stanford Memorial Church. In 2008, Gregg appeared in the film
Iron Man as
S.H.I.E.L.D. agent
Phil Coulson. In 2010, he reprised his role as Agent Coulson for
Iron Man 2. Gregg had since signed up for a multiple film deal as the character with
Marvel Studios. In 2011, he returned again as Coulson for
Thor. Gregg noted his being a part of the expanding
Marvel Cinematic Universe as being very exciting; "Agent Coulson was one of the guys who wasn't really in the comic books, and he was a very kind of small role in
Iron Man," he said, "and I was just very lucky that they chose to expand that character and chose to put him more into the universe of it. It's really a blast!" Following on from his appearance in
Thor, he again reprised his role in
The Avengers. Gregg stars in a series of short films,
Marvel One-Shots, that center around his character and can be seen on the Blu-ray releases of the films. In October 2010, Gregg was part of the cast of a staged reading of
Larry Kramer's
The Normal Heart alongside
Dylan Walsh,
Lisa Kudrow, and
Tate Donovan, presented in Los Angeles on the occasion of the play's 25th anniversary (and preceding the play's
2011 Broadway premiere, which retained elements of this staged reading); the reading was directed by his then-father-in-law,
Joel Grey. From 2013 to 2020, Gregg portrayed Agent Phil Coulson in the ABC television superhero drama series
Agents of S.H.I.E.L.D., set within the MCU, alongside
Ming-Na Wen and
Chloe Bennet. He directed episodes in seasons five and six. On April 20, 2013,
Trust Me, a film written, produced, and directed by Gregg, premiered at the
Tribeca Film Festival. The film found limited release in the United States in June 2014. Gregg reprised his role as Agent Coulson in the 2019 superhero film
Captain Marvel and the animated series
What If...?, episodes "
What If... the World Lost Its Mightiest Heroes?" and "
What If... Thor Were an Only Child?", released in 2021. In 2022, he joined the
TNT series
Snowpiercer for its fourth and final season. However, the series was cancelled by the network before it could air there. The fourth season was picked up by
AMC, and aired in 2024. ==Personal life==