Born in Austin, Texas, Forbes hoped to become a ballet Dancer. She began receiving formal acting training at the High School for the Performing and Visual Arts in Houston. While on vacation in New York City at the age of 16, she found herself auditioning for a film; although she was not selected, she signed with the william Morris Agency and began her professional acting career. In 1987, at age 22, she 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.

Although she has appeared in significant roles in movies such as Escape from L.A., Kalifornia and Swimming with Sharks, Forbes is known for her recurring appearances on genre and drama shows such as Ensign Ro Laren in Star Trek: The Next Generation and her regular role as medical examiner Julianna Cox on Homicide: Life on the Street during the 1990s, while building her career with recurring roles throughout the 2000s in Battlestar Galactica, 24, In Treatment, Durham County, Prison Break and her series regular role as Maryann Forrester on True Blood.

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 the 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. She continued performing on television during this period, with guest spots on Seinfeld, The Outer Limits, and Star Trek: The Next Generation, to which she returned to tie up the Ro Laren storyline in the series' penultimate episode.

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. That same year, Forbes became a regular on Wonderland, an ABC series, but it was pulled from the air after only two episodes. Forbes was next seen in a recurring role on the police drama The District.

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. Actor Dennis Haysbert, who played President Palmer, reportedly was a big Star Trek fan and excited to work with her given her history with the show. She followed her role on 24 with a guest spot on Alias, then went on to play Admiral Helena Cain in three episodes of the re-imagining of the classic sci-fi series Battlestar Galactica, as well as starring in the TV movie Battlestar Galactica: Razor. She followed this up with a recurring role as Agent Samantha Brinker on the drama Prison Break and guest starred on Boston Legal and Lost.

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 teen 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.