In 1987, at age 22, Forbes landed the dual roles of Solita Carrera and Sonni Carrera Lewis on the daytime soap opera Guiding Light. She performed on the show for two years, receiving a Daytime Emmy Award nomination for her performance in 1990. After this role, she continued in theater, which was an early love of hers, and began appearing in small guest roles on television to raise her profile. She went on to make guest appearances on a few other TV shows (including Star Trek: The Next Generation and Father Dowling Mysteries) before landing the recurring role of Ensign Ro Laren, a fiery, yet reserved Bajoran, in Star Trek: The Next Generation.

Forbes married actor Ross Kettle in 1990; the marriage ended in divorce.

The producers of Star Trek invited Forbes to reprise Ro in the spin-off series Star Trek: Deep Space Nine, but Forbes declined the offer and decided to focus on a career in films. She received praise, as well as a Saturn Award nomination from the Academy of Science Fiction, Fantasy & Horror Films, for her performance as photographer Carrie Laughlin in the 1993 thriller Kalifornia. This was followed with the lead female role in the 1994 black comedy Swimming with Sharks, as well as supporting roles in such films as The Road Killers, Just Looking and John Carpenter’s 1996 science fiction/action sequel Escape from L.A.

In 1996, Forbes joined the cast of NBC’s popular police drama Homicide: Life on the Street, playing chief medical examiner Julianna Cox. She remained with the show for two years, then was let go as part of a major cast overhaul (the series would be canceled after one more season). However, she would reprise her role in the 2000 TV special Homicide: The Movie.

She was subsequently given roles in films such as 2001’s Perfume and 2002’s American Girl, as well as the 2001 British TV movie Messiah (for which she studied British sign language for a week) and its sequel installments: Messiah 2: Vengeance is Mine in 2002 and Messiah III in 2003. During the 2002–2003 TV season, Forbes played the recurring character of presidential aide Lynne Kresge on the second season of the action series 24.

Forbes returned to British television screens with guest roles in both Holby City and as a Mossad agent in Waking the Dead. In 2008, Forbes starred in two HBO drama series; In Treatment, portraying the wife of the central character, and in True Blood as a Maenad named Maryann Forrester.

She reprised her semi-regular roles in both series in their respective second seasons in 2009, as well as joining the cast of the Canadian psychological drama series Durham County for its second season as Dr. Penelope Verity.

Forbes has been a vegetarian since she was a teenager and became vegan in 2011.

Forbes subsequently played Valerie Edwards in the TV series Berlin Station that premiered on Epix on October 16, 2016. Her character is the no-nonsense administrator who serves as a Berlin Station Internal Branch Chief. A ten-episode second season is set to premiere on October 15, 2017.