Her father, John, was a meat salesman, which required considerable travel and resulted in frequent moves. Her mother, Maxine, left the family when Jill was small. She also has a younger brother, John Jr.
Described by Miguel Ferrer as being a non-angry version of her "Crossing Jordan" (2001) character Jordan Cavanaugh.
Voted one of the "Fun and Fearless Females" by Cosmopolitan Magazine (2002.)
Had a second marriage ceremony in January 2001 at New York City Hall with 'Rudolph Giuliani' officiating.
She used to busk (play the guitar and sing for money) in the New York City subway.
Sometimes makes appearances at music events as a guest guitarist where she usually performs barefoot.
She is such a fan of the folk duo Indigo Girls that a poster of the act hangs in the kitchen set of her character in "Crossing Jordan" (2001).
She and Paolo welcomed a son, Marco, on 17 September 2003.
Finalist for role of Dana Scully on _"The X-Files" (1993)_ .
Twin sister of Jacqueline Hennessy
Speaks Italian, French, Spanish and German
Has a song named after her: "The Ballad of Jill Hennessy" by Mollycuddle.
Graduated from Kitchener, Ontario, Canada's Grand River Collegiate (high school).