Preston was born as Kelly Kamalelehua Smith  in Honolulu, Hawaii. Her mother, Linda, was an administrator of a mental health center, and her father, who worked for an agricultural firm, drowned when Preston was three years old. Her mother married Peter Palzis, a personnel director, who subsequently adopted her; she used his name when she began her acting career. Her stepfather was Lee Carlson. She has a younger maternal half-brother, Chris Palzis.[As a child, she spent time living in Iraq and Australia. While living in Australia, she attended Pembroke School in Adelaide, South Australia. She later attended Punahou School and studied drama and theater at the University of Southern California.