Kristin Cavallari was born in Denver, Colorado, the second of two children of Judith Spies (née Eifrig) and Dennis Cavallari.
When her mother divorced her father, her older brother, Mike, moved with their father to Laguna Beach, California, while she moved with her mother to the village of Barrington, Illinois, a suburb an hour northwest of Chicago. She attended Barrington Station Middle School, and graduated on to Barrington High School. After difficulties adjusting to a new life with a stepfather and stepbrother, Cavallari moved to California to live with her father. There, Cavallari was enrolled at Santa Margarita Catholic High School for her sophomore year. After Kristin Cavallari attended a Driver's Education course through Laguna Beach High School, her father enrolled her in Laguna Beach High School.