O'Mara performed with
The Royal Shakespeare Company. His theatre work in London and Dublin included
The Jew of Malta and
Popcorn, and he was nominated for Best Supporting Actor in 2002 at the Irish Theatre Awards for his portrayal of John in
Neil LaBute's
Bash. He appeared in
Harold Pinter's
The Homecoming in London and Dublin as well as Lincoln Center in New York. He had lead roles in other series, including
The Agency,
Band of Brothers,
Monarch of the Glen,
High Stakes,
Playing the Field,
The Bill,
Berkeley Square, and
Reach for the Moon. He also played Jim Shannon, the lead role in the 2011
Fox TV series
Terra Nova. He played
Philip Marlowe in a 2007 pilot entitled
Marlowe; that same year, he portrayed video game character Albert Wesker in
Resident Evil: Extinction, the third film in the live-action Resident Evil film franchise, and the second sequel to
Resident Evil. O'Mara also played the role of
George Washington in the
History Channel miniseries
Sons of Liberty (2015). In both 2005 and 2008, O'Mara guest-starred on TNT's
The Closer as Bill Croelick, a charming and psychotic man with a fixation on fire. O'Mara also guest-starred in the
Criminal Minds season two episode "The Last Word" as a serial killer. O'Mara appeared in
One for the Money as Joe Morelli. He also appeared on
Vegas as Jack Lamb. O'Mara joined the cast of
Agents of S.H.I.E.L.D., where he portrayed
Jeffrey Mace, the new director of
S.H.I.E.L.D. ==Personal life==