Her identical twin sister, Jacqueline, is a magazine writer and TV show host in Canada. She and her sister played twin prostitutes in Dead Ringers.

Hennessy was born in Edmonton, Alberta, and graduated from Grand River Collegiate Institute in Kitchener, Ontario. Hennessy's father, John, was a meat salesman, which required considerable travel and resulted in frequent moves. Her mother, Maxine, left the family. She also has a younger brother, John Hennessy, Jr.

Hennessy married Paolo Mastropietro, a former bartender, on October 1, 2000. Her husband opened a tavern in Northvale, New Jersey in 1999, and named it "Hennessy's Tavern", after his wife. The tavern serves comfort food. 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 Rudolph Giuliani officiating. Their son, Marco, was born on September 17, 2003.

Hennessy is most famous for playing ADA Claire Kincaid on Law & Order and the lead role as Jordan Cavanaugh on Crossing Jordan. She is multi-lingual, being able to speak Italian, French, Spanish, and German. This is partly because of her mixed heritage, which includes Irish, Italian, Swedish, French, and Ukrainian.

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.

    * She used to busk (play the guitar and sing for money) in the New York City Subway.
    * She sometimes makes appearances at music events as a guest guitarist where she usually performs barefoot.
    * Hennessy is a fan of the Indigo Girls and a poster of the act hangs in the kitchen of Jordan's on-screen apartment.
    * She was short-listed for the role of Dana Scully, on The X-Files.

    * Jill Hennessy at the Internet Movie Database.