Armstrong's professional acting career began in 1975 with the Off-Off Broadway debut, Harmony House. Then, in 1977, Armstrong made her television debut as Julia Peters on the CBS sitcom, On Our Own. In 1978 Armstrong starred opposite Richard Thomas in her first TV-movie Getting Married. She co-starred again with Richard Thomas in a 1981 stage production in Seattle of Neil Simon's Barefoot in the Park, from which a video was made for HBO broadcast that year.
Armstrong continued to make several films for both the big and small screens in the 1980s, among them High Road to China opposite Tom Selleck; Jaws 3-D with Dennis Quaid; Alan Alda's The Four Seasons; the TV miniseries Lace; and Nothing in Common, starring Tom Hanks and Jackie Gleason.
The 1990s brought Armstrong to her best-known role, playing Patty Chase on the critically acclaimed series My So-Called Life. She later starred in several television films. In 2000, she appeared on the NBC sitcom Frasier, in the episode "Mary Christmas." In 2008, Armstrong played Penelope Kendall on ABC's Boston Legal. Armstrong remains active in films, television, and the stage. She had a recurring role in the Showtime series House of Lies as Julianne Hotschragar. She also appeared in Castle, Mad Men and NCIS.
