Post's early acting credits included a 1979 episode of Barnaby Jones and the pilot episode of Simon & Simon "Details at Eleven" in 1981, episode one of season two of The Greatest American Hero, two episodes of The A-Team as two different characters in the 1983 episode "The Only Church in Town" and the 1984 episode "Hot Styles," and The Love Boat. She appeared in the science fiction series Buck Rogers in the 25th Century and as Diane Chambers' best friend in the sitcom Cheers, before eventually becoming a regular on the ABC action drama The Fall Guy. After The Fall Guy, she played Christine Sullivan on the 1980s television comedy series Night Court from the third season until the show's end. She played Georgie Anne Lahti Hartman on the comedy series Hearts Afire, co-starring John Ritter. Post also had regularly recurring guest star roles on The District and on Scrubs as the mother of Dr. Elliot Reid.
Film credits include There's Something About Mary (1998), in which Post played Mary's mother. She played a call girl and dominatrix in the 1988 TV movie Tricks of the Trade opposite Cindy Williams, and a singer in Glitz with Jimmy Smits, based on the novel by Elmore Leonard. She also had a starring role in NBC's 1995 movie Visitors in the Night. She appeared as reporter Christine Merriweather in the 2007 improvisational comedy film (released in 2017) Cook Off!. She appeared in the 30 Rock episode "The One with the Cast of Night Court" playing herself when she, Harry Anderson, and Charles Robinson staged a mock reunion of the Night Court cast. Post was the voice of June Darby on the computer animated robot superhero TV series Transformers: Prime. She appeared as recurring character Barbara 'Bunny' Fletcher in the first four seasons of Chicago P.D.