Farrell starred as Lieutenant Commander Jadzia Dax, Deep Space 9'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. Although she played the character for the series' first six years, she made the decision to leave 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 the television comedy series Becker from Paramount. Her role was the one of tough but good-hearted local diner owner Regina "Reggie" Kostas. She appeared in Becker for four years and 94 episodes. Rumors were that contract disputes arose, and at Farrell's request to fight for more pay, the producers used her as a scapegoat, and she was unceremoniously let go from her Paramount contract. She was replaced by actress Nancy Travis.