Best known for his roles in such films as Lucky, Coherence, Roman J. Israel, Esq., and Daniel Martinico’s OK, Good and Excursions, Armstrong's television work includes recurring roles on True Lies, Bosch: Legacy and NCIS as well as The Blacklist, Room 104, and Fear the Walking Dead. In theater, he recently appeared in Evanston Salt Costs Climbing at Rogue Machine Theatre. He is also known for his onstage roles in The Ants at the Geffen Playhouse, Twilight: Los Angeles, 1992 at the Mark Taper Forum, Uncle Vanya at Pasadena Playhouse, and Power of Sail also at the Geffen Playhouse.

Recent VO includes podcast series DUST, Agent Stoker, and Blood Weed on Audible Originals, as well as voices for Cinema Toast.