Mimsy Farmer first began acting at age 16 when a press agent noticed her and offered her work in film which she accepted. Her first film was a minor part in Spencer's Mountain (1963). After her first acting role, Mimsy took acting lessons after graduation and landed a few more roles playing secondary characters. After spending a year in Canada and working in a research hospital, she returned to the USA, moved to Los Angeles, and was soon cast for a role in Roger Corman's The Wild Racers (1968), which was directed by David Haller. Her experience on that film was to her 'a pleasant one' because she first traveled to Europe and experience the various countries and to England to visit her older brother who worked as a math teacher at a university in London.
After appearing in the film More (1969), Mimsy traveled to Italy for a vacation and met her future husband, screenwriter Vincenzo Cerami, who wanted to write her a part in a film. He was later fired as the scriptwriter and her role was not cast. After spending time in Italy, and disillusioned by the civil unrest and political problems with the USA and its involvement in the Vietnam War, Mimsy, a liberal left-winger, settled in Italy to continue her acting career there.
Mimsy Farmer first became an international star when Dario Argento cast her to appear alongside Michael Brandon in 'giallo' mystery-thriller Four Flies on Grey Velvet in 1971. After her success with Four Flies on Grey Velvet, Mimsy remained in Italy and a steady stream of acting roles followed with dramatic parts in dramas and thrillers including Allonsanfan, and Il profumo della signora in nero directed by Franceso Barilli. One of her best roles was a starring role in the horror-mystery-thriller Macchie solari (Autopsy), directed by Armando Crispino, where she played a pathologist investigating a murder.
She also appeared in two films directed by Ruggero Deodato titled Concord Affaire (1979) and Camping del terrore (1987). Lucio Fulci even cast her in 1981 for a co-staring part in Gatto Nero (The Black Cat) (1981) playing the heroine/victim. She also appeared in a number of French language films and TV. After her divorce from Vincenzo Cerami in the 1980s, Mimsy and her teenage daughter settled in France where she also did some French-language movie and TV roles and she considers French an easier language to learn and speak than Italian.