Known best for his work in the Star Trek television shows, Armstrong played various Klingon, Cardassian, Romulan, and human characters in Star Trek: The Next Generation; Star Trek: Voyager; Star Trek: Deep Space Nine; and Star Trek: Enterprise.
Before beginning his acting career, he served in the Vietnam War.
He appeared in two 1996 episodes of Babylon 5 and in one episode of Seinfeld. In addition, he and several of his Star Trek castmates formed a music group called The Enterprise Blues Band.
He was born and raised in Sonora, California.
He and Scott Bakula appeared together in both Star Trek: Enterprise and in an episode of Quantum Leap.