Born in Edmonton, she moved and lived in Kitchener Ontario.Hennessy is famous for playing Claire Kincaid on Law & Order (1993 – 1996) and the lead role as Jordan Cavanaugh on Crossing Jordan. In 1999 she played Lisa in Chutney Popcorn. In 2000, Hennessy wrote, produced, and co-directed an independent film called The Acting Class. Inspired by a true story, the mockumentary looks at the trials and tribulations of a dysfunctional acting class. The film co-starred her sister, and included cameos by a number of Hennessy’s former Law & Order co-stars.In 2001 she portrayed Jackie Kennedy in the film Jackie, Ethel, Joan: The Women of Camelot. In 2003 she made a cameo appearance in the film Abby Singer. She also appeared on Broadway in the musical Buddy – The Buddy Holly Story in 1990. Most recently, she portrayed Tim Allen’s wife in the 2007 movie Wild Hogs.
On June 9, 2007, she received a star on Canada’s Walk of Fame. She also has been recording an album in Austin, Texas.