Jill Hennessy is a Canadian actress, known for her television roles on Law & Order and Crossing Jordan. She was born in Edmonton, Alberta. Her father, John Hennessy, was a meat salesman, a job that required considerable travel and resulted in frequent moves for the family. Her mother, Maxine, left the family in 1982, leaving her daughter to be partially raised by her paternal grandmother. Hennessy has Irish, French, Swedish and Italian ancestry on her father's side, and mostly Ukrainian, as well as Austrian, ancestry on her mother's. She graduated from Grand River Collegiate Institute in Kitchener, Ontario.

Hennessy and her sister played twin call girls in 1988's Dead Ringers. She was short-listed for the role of Dana Scully, on The X-Files. 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 with Elizabeth Holder 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, and 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.