Knox was born in Hartford, Connecticut, the daughter of Austrian immigrants Hermine Sophie (née Muck) and Frederick Kornbrath.

She appeared as herself in the Universal Studios 1944 production Follow the Boys, one of the World War II morale-booster films made both for the soldiers serving overseas and civilians at home.

Knox also was a pin-up girl during the war, appearing in such magazines as Yank, a weekly published and distributed by the United States military.

Based on the very popular comic strip, the instant success of the May 1946 film led to Knox appearing in another five Joe Palooka productions.

Kristin became an actress and painter, who at 17 married recording artist Ricky Nelson and gave birth to four children: Tracy, twins Gunnar and Matthew, and Sam.