Armstrong began acting in his native Michigan, where he founded his own theater company. By the early 1980s, he was touring with "Da" and appearing in regional theaters around the country.
Armstrong made his film debut as Tom Cruise's arrogant pal in "Risky Business" (1983), and the following year created the role of "Booger" in the unexpectedly popular comedy "Revenge of the Nerds". He went on to re-create "Booger" in "Revenge of the Nerds II" (1987) and the TV sequels "Revenge of the Nerds III: The Next Generation" (Fox, 1992) and "Revenge of the Nerds IV: Nerds in Love" (Fox, 1994). Thus far, "Booger" has proven to be Armstrong's most lasting film image. His other appearances include the teen comedies "Better Off Dead" and "Bad Medicine" (both 1985), a Neanderthal named "Goov" in "Clan of the Cave Bear" (1986), a radio operator in "Hot Shots: Part Deux" (1993), Country Jake in "The Adventures of Huck Finn" (1993) and "Big Bully" (1996). Armstrong continued to rack up numerous film credits, in comedies honoring his iconic 80s films such as "Van Wilder" (2002) and "Dodgeball: A True Underdog Story" (2004), and in dramas; the actor was particularly enjoyable in his turn as music mogul Ahmet Ertegun in the Ray Charles biopic "Ray" (2004).
"Moonlighting" was Armstrong's TV debut (he joined the cast in the second season). He later appeared in the TV-movies "Hi, Honey, I'm Dead", as a reincarnated realtor, (Fox, 1991), the "America's Most Wanted" spoof "Public Enemy Number 2" (Showtime, 1991) and a children's special based on a William Saroyan story, "The Parsley Garden" (ABC, 1993). He has also made guest appearances on the series "Grand" and "Murphy Brown". More recently the actor could be seen in recurring roles on "Felicity," "Ed" and "One on One."