Walsh was born April 11, 1947 in Kenton County, Kentucky, United States, North America to Martha and Thomas Walsh (died 1965). Her father, Thomas Walsh, who was President of the Atlantic Underwriters Agency, was killed in the November 8, 1965 crash of American Airlines Flight 383 in Boone County, Kentucky. Walsh was a graduate of the Royal Academy of Dramatic Art in London, England, and attended the University of London.
Walsh made her acting debut in the 1966 film The Chase. She went on to guest star in various shows such as The Monkees, The Virginian, and Daniel Boone.
Walsh is best known for the last professional acting assignment of her career, as Lulu in The Trip (1967), which was written by Jack Nicholson and directed by Roger Corman.
On October 7, 1970, Walsh was found dead in her flat in Kensington, an area in London, England. Although her death was cited as murder, no details have ever been known to be released, and the case remains unsolved (as of 2016). The Coroner's report in London returned an open verdict because of insufficient evidence stating that "Kathy died of alcoholic and barbiturate poisoning but it had been impossible to determine whether it was suicide or accidental."