Campbell married Canadian actor Jeff Colt on April 3, 1995; the couple, who met when he was a bartender at Toronto's Pantages Theatre, divorced on May 8, 1998. She has also dated actors John Cusack, Matthew Lillard, and Pat Mastroianni. Campbell appears in campaign literature and videos for the Tourette Syndrome Foundation of Canada and the Tourette Syndrome Association, a similar organisation in the United States. Her younger half-brother, Damian McDonald, has Tourette syndrome. She also has an elder brother, Christian, and another brother, Alex, both of whom are actors.

In 2005, Campbell began dating John Light, an English actor who she met while filming the movie Investigating Sex. The couple became engaged in December 2005 and live in London, England. When Light proposed, he got down on one knee and recited Shakespeare to Campbell. They married in Malibu on May 5, 2007; her father gave away the bride.