Born in Wisconsin, Casey settled with her family in Texas in 1980. After attending the University of Houston, where she earned a degree in journalism. In 1984 Casey kicked off her writing career as an intern at the now-defunct Houston City Magazine. When she left two years later, she was a senior editor. Casey's next stop was a two-year interlude as editor of Ultra, a state-wide magazine geared toward wealthy Texans.
In the years after Ultra, Casey branched out to a national audience, her articles appearing in Ladies' Home Journal, where she was a contributing editor for 18 years, as well as More , TV Guide, Rolling Stone, Seventeen, Reader's Digest, and Texas Monthly. During her more than two decades as a magazine journalist, Casey interviewed celebrities, including movie, television, and recording stars, presidents and first ladies. She covered subjects that ranged from the Oklahoma City bombing, the aftermath of 9/11, and Hurricane Katrina, to infertility and the McCaughey Septuplets.
Throughout those years, many of Casey's articles examined sensational crimes. In the early 1990s, she took one such case, the subject of a Ladies' Home Journal article on a serial rapist attacking Houston-area women, and turned it into her first book, The Rapist's Wife. After its publication, Casey moved away from magazine writing and concentrated on books, at first true crime books, non-fiction accounts of actual cases. Perhaps her most widely read book, She Wanted It All, recounts the case of Celeste Beard, who married an Austin multimillionaire, only to convince her lesbian lover, Tracey Tarlton, to kill him.
Author Ann Rule has called Kathryn Casey "one of the best in the true crime genre."
Based on Casey's experience covering real crimes, she turned her attention to crime fiction. In 2009, "Booklist," the publication of the American Library Association, named Casey's first novel, Singularity, one of the "Best Crime Novel Debuts of 2009" on its Bestseller Lists. The main character in Casey's mystery series is Sarah Armstrong, a Texas Ranger and a criminal profiler. In Singularity, Armstrong travels across Texas hunting a deviant serial killer.
In addition, Casey is a co-founder of and a regular contributor to Women in Crime Ink, which has been described by the Wall Street Journal as "a blog worth reading."
In 1995, Casey appeared on The Oprah Winfrey Show to discuss The Rapist's Wife along with the woman referred to in the title, Linda Bergstrom. In the years since, Casey has appeared on Oprah Winfrey's Oxygen Network, Court TV, Biography, Nancy Grace, E! Network, Investigation Discovery and the A&E.