Kozak was born in Wilkes-Barre, Pennsylvania, the daughter of Dorothy (née Taraldsen), a university music teacher, and Joseph Aloysius Kozak, an attorney. She has seven older siblings. Their father died when Harley was a year old. She was raised in Nebraska and attended New York University's Graduate Acting Program at the Tisch School of the Arts, graduating in 1980.
Kozak starred in movies such as Necessary Roughness, The Favor, Parenthood, Arachnophobia, All I Want for Christmas and the soap operas Texas (from November 1981 to December 1982 as Brette Wheeler), Guiding Light (from 1983 to 1985 and a one day voice over in February 1990, as both Annabelle Sims Reardon and Annabelle's deceased mother, in 1983 flashbacks, Annie Sims) and Santa Barbara (from 1985 to 1986 and again in 1989, as former nun, Mary Duvall McCormick). In Santa Barbara her character died in an accident where a giant neon letter "C" toppled on top of her during an argument atop the Capwell hotel. Viewers were so angry over Mary's death that they started a letter-writing campaign demanding for her reappearance. The show received such huge amounts of letters that eventually they admitted their mistake and asked Kozak to come back. She declined the offer since she was already working with other projects and she was proud of the unusual way her character had made her exit. In February 1989, she made a brief return though, as an angel in Heaven. In 1987 Harley received a Soap Opera Digest Award from her role as Mary.
In 1993–1994, Kozak played Alison Hart, wife of Dave Hart, portrayed by Beau Bridges, on the CBS comedy/western series Harts of the West. The program was set on a dude ranch in Nevada. Lloyd Bridges played her father-in-law, and Diane Ladd appeared once as Alison's mother. Her three children were played by Sean Murray, Nathan Watt and Meghann Haldeman, who has been friends with her on-screen mother ever since. Also Mark Harmon guest-starred the series. Kozak has worked several times with Harmon, both in movies and TV series and she credits him as one of her favorite co-stars.
In 1996, Kozak appeared on mini-series Titanic along with Peter Gallagher and Catherine Zeta Jones. She played the role of Bess Allison, a mother who dies while searching her missing baby at the time when Titanic sinks. Kozak has stated that she has never been able to watch her performance, because the role was simply too sad.
Kozak was originally chosen to play Karen Sammler in the TV show Once and Again. She filmed the pilot, but was asked to withdraw from the series when she became pregnant with her first child. The role went to Susanna Thompson. Kozak also appeared in the 1st year episode "Cold Lazarus" of Stargate SG-1 as Sara O'Neill.
Kozak has written five novels: Dating Dead Men (2004), Dating Is Murder : A Novel (2005), Dead Ex (2007), and A Date You Can't Refuse (2008) all of which feature greeting card designer and amateur sleuth Wollie Shelley, a woman with very eccentric friends and family. The first three novels were published by Doubleday, a division of Random House, the most recent was published by Broadway Books following the restructuring of Doubleday. Dating Dead Men won an Agatha Award, an Anthony Award, and a Macavity Award for best first novel. Kozak’s fifth book is Keeper of the Moon, a paranormal romantic suspense novel.
Kozak lives in Los Angeles, California. She has been married twice: a brief union in the early 1980s and a second marriage from 1997 to 2007 with entertainment lawyer Gregory Aldisert, with whom she shares three children.