Harmon was nominated for an Emmy for his work on The West Wing. He went on to star in NCIS. He had previously had starring roles on Chicago Hope and Reasonable Doubts. The latter also featured recurring West Wing guest star Marlee Matlin.
His role in The West Wing was not his only performance set in the White House. In the 2004 romantic comedy Chasing Liberty, he played President James Foster.
He currently plays NCIS Special Agent Leroy Jethro Gibbs on "NCIS", a show which he is also a Producer and later an Executive Producer of.