Before becoming a model, she played a Christmas elf at a mall in Cedar Rapids. In 1978, at age fifteen, she left her hometown for a summer in Mexico City as a foreign exchange student. That time in her life made her fond of big cities, so in her junior year of high school, the nearly six feet tall Farrell submitted her image to the Elite modeling agency in New York City. Shortly after, at the age of sixteen, she was summoned to NYC and, within two days of arriving, had an exclusive contract with Mademoiselle magazine.

After eighteen months of modeling, she studied acting with Kate McGregor Stewart while still modeling on the side. Her first major roles were in the short-lived 1983 television series Paper Dolls playing a model and in the feature film Back to School with Rodney Dangerfield. In the spring of 1989, she began studying acting with Stella Adler and appeared in a number of guest-starring roles in series like Quantum Leap and The Cosby Show. In 1992, she played Cat in a second pilot for a U.S. version of Red Dwarf, which was not picked up.

A year later, she was offered a lead role in Star Trek: Deep Space Nine: Farrell starred as Jadzia Dax, Deep Space Nine`s Starfleet science officer; a character who is host to a 300-year-old symbiont and can draw upon the memories and knowledge of the symbiont`s six previous hosts. She left the show at the end of the sixth season; as a result, Paramount Pictures decided to kill off Farrell`s character.

Farrell then co-starred on Paramount`s television comedy series Becker. She played the tough but good-hearted local diner owner Regina "Reggie" Kostas for four years and 94 episodes, before she was replaced by Nancy Travis.