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. Although it is widely reported that her middle name is Sally (also the name of her grandmother), Sally is actually her birth name while Vanessa is her middle.

Marcil has occasionally denied that she married actor Corey Feldman^[1], but Las Vegas marriage records show that the two married in a legal ceremony on August 6, 1989.^[2] 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).^[3] They have a son, Kassius Lijah Marcil-Green, born on March 30, 2002. She is currently living with her boyfriend Ben Younger and her son in Los Angeles.