Heather Menzies-Urich, her co-star in The Sound of Music (1965), eventually was godmother to her son. They had become close friends despite the fact that while filming that movie's rowboat scene, Kym swallowed too much water and threw up on Heather.
Both parents were Greek. The family's name was originally Karathanasis until her father shortened it to Karath.
Former sister-in-law of Alan Callow.
Graduated from USC with a degree in Humanities
Has a son, Eric, born in 1991. Her The Sound of Music (1965) co-star, Heather Menzies-Urich, is his godmother.
Has twice portrayed a younger sister of a character played by a Cartwright sister, both movies which featured large families. She played Pattie-Cake Spencer to Veronica Cartwright's Becky Spencer, two of nine siblings, in Spencer's Mountain (1963). And she played Gretl Von Trapp to Angela Cartwright's Brigitta Von Trapp, two of seven siblings, in The Sound of Music (1965).
Kym's older sister, Frances Karath, auditioned for the role of "Liesl von Trapp" in The Sound of Music (1965).
Lives in New York.