She's born in Edmonton, Alberta (november 25, 1968).
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, Eleanor, in Kitchener ontario.
She has a younger brother, John Hennessy Jr. and an identical twin sister Jacqueline who is a magazine writer and TV show host in Canada.
Hennessy has Irish, French, Swedish and Italian ancestry on her father's side, and mostly Gypsy Ukrainian, as well as Austrian, ancestry on her mother's.
She graduated from Grand River Collegiate Institute in Kitchener, Ontario and used to busk in the Toronto Subway.

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 an independent film called The Acting Class. Inspired by a true story, the comedic faux documentary 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.

Hennessy is multi-lingual, being able to speak Italian, French, Spanish, and German. She married Paolo Mastropietro, a former bartender and sometime actor, on October 1, 2000. Her husband opened a tavern in Northvale, New Jersey in 1999, and named it Hennessy Tavern, after his wife. The tavern is located next to the former bowling alley where the television series Ed was filmed. Hennessy also enjoys riding motorcycles with her husband. They also had a second marriage ceremony in January 2001, which took place at New York City Hall with Mayor Rudolph Giuliani officiating. Their son, Marco, was born on September 17, 2003. The couple is currently expecting their second child, a boy, due in winter 2007/2008.
Jill is a fan of the Indigo Girls and a poster of the act hangs in the kitchen of Jordan's on-screen apartment. She sometimes makes appearances at music events as a guest guitarist where she usually performs barefoot.
The Minneapolis-based band Mollycuddle wrote the song "The Ballad of Jill Hennessy" in honor of the actress. Jill was reportedly very pleased, and offered to perform with the band on rhythm guitar.