Karath started her career at the age of three, appearing in Spencer's Mountain with Henry Fonda and Veronica Cartwright in 1963 (she would subsequently work with Veronica's younger sister Angela Cartwright in The Sound of Music). In 1964 she was seen in The Thrill of It All with Doris Day and Good Neighbor Sam with Jack Lemmon.

Karath is best remembered for her role in The Sound of Music, where she played the "baby of the family," Gretl Von Trapp. She won her role when she marched in to meet Robert Wise the director, with a portfolio under her arm and declared: "I've been in three films already, I'm exactly who you're looking for."

When on the Ellen Show, Julie Andrews was asked about working with the Von Trapp children. Julie replied, "They were all lovely. However, the youngest one (Karath) was probably the most difficult for me, because she was just a tad heavy in those days. Today, she is this amazing gorgeous looking Monroe-esque young lady."

After The Sound of Music Karath primarily appeared in American television, including appearances in The Brady Bunch and Lost in Space.

Kym remains good friends with the other 'Von Trapp' children today, and in 2000 she went back to Salzburg with them, filming a documentary about the making of the film.

Although Karath was born in the States, her family was actually Greek. Her original surname was Karathanasis, but then it was changed to Karath by her father. She has an older sister, Francie, who auditioned for the eldest Von Trapp child in The Sound of Music, a role that eventually went to Charmian Carr.

Karath graduated from the University of Southern California with a degree in Humanities, and shortly after that she moved to Paris, France where she studied art history and modelled as well. At the age of 26, she married Philippe L Equibec; they have a son, Eric.

Karath has said she still isn't fond of the water after nearly drowning in the boat scene in The Sound of Music as she couldn't swim.