Marcil was born to Peter (of Mexican descent) and Patricia (of French, Italian and Portuguese descent), and has three siblings. She was named after actress Vanessa Redgrave.

She grew up in poverty and her father was an alcoholic and abusive. Marcil herself had problems with alcohol and drugs while she was a teen and went to juvenile hall after being arrested several times. She dropped out of high school briefly, but eventually graduated and moved to Newport Beach, California to live on her own. She struggled financially before deciding to become an actor and taking acting classes.

Marcil acted in a number of theatre productions before landing the role of Brenda Barrett on the soap opera General Hospital in 1992. She garnered three Daytime Emmy Award nominations (1997, 1998 and 2003) for her portrayal, winning in 2003 as "Outstanding Supporting Actress." In February 1998, she was named "Outstanding Lead Actress" at the Soap Opera Digest Awards.

After six years on General Hospital, she left the show in 1998 (returning briefly in '03) to star in the made-for-television movie To Love, Honor and Deceive, and had a recurring guest role on the police drama High Incident produced by Steven Spielberg. Marcil joined the cast of Beverly Hills 90210 in November 1998 as Gina Kincaid and remained with the show for one-and-a-half seasons.

Marcil made her feature film debut in the 1996 film The Rock, in which she starred opposite Nicolas Cage, Sean Connery and Ed Harris. In 1999, she starred in two independent films: Nice Guys Sleep Alone with Sean O'Bryan and This Space Between Us with Jeremy Sisto.

Marcil was awarded the 2001 "Sojourn Service Award" and has supported Sojourn Services for Battered Women and their children by hosting charity events and making appearances on Wheel of Fortune, playing for funds for the organization. Through her fundraising efforts, she hopes to increase awareness of domestic violence and inspire battered woman to take control of their lives.

Marcil currently stars in the television series Las Vegas. She's been featured in several Men's magazines, including Stuff, FHM, and Maxim. She was named #19 on Maxim's Hot 100 of 2005 list and was featured on the cover of the issue that included the list.[1]

Marcil was rumored to have married actor Corey Feldman in 1989, but she has denied this in interviews. She was engaged to General Hospital co-star Tyler Christopher, but the two never married. She dated former Beverly Hills 90210 castmate Brian Austin Green until early 2003 (they were widely reported to have been engaged, but Marcil denied this in an interview with Stuff magazine). They have a son, Kassius Lijah Marcil-Green, born on March 30, 2002. She currently lives with her son in Los Angeles.