Lived with her parents in Colorado, until her parents divorced. Her dad moved to Laguna Beach, California with her brother, and she moved to Illinois with her mother. She also has a stepfather and stepbrother there. She acted out while there to get attention and, after her mother couldn't handle it anymore, Kristin moved to Laguna Beach to be with her dad and brother. Her freshman year, she attended a Catholic school with the uniforms to boot. But that year, she took Driver's Ed at Laguna High.. where she met Alex Murrel (aka Alex M.) and soon the rest of the "Laguna Beach: The Real Orange County" (2004) crew. She convinced her Dad to let her switch to the public school starting her sophomore year.
At the time, she was involved in an on-and-off relationship with senior Stephen Colletti. Cavallari's romance with Colletti caused a rivalry with another cast member, Lauren Conrad. The love triangle became one of the series' central plotlines.
In May 2009, Cavallari confirmed that she would be joining the cast of the popular MTV reality series The Hills. She appeared in the mid-season finale that aired on May 31, 2009 and appeared as a regular in the remainder of the fifth season. She inked a deal with MTV to appear as a series regular throughout two more seasons of the series following the fifth. The sixth and final season aired summer 2010.
Cavallari dated Stephen Colletti during their high school days, as seen on Laguna Beach. Not long after, she dated her future Hills co-star Brody Jenner. After taking a break from reality television, she was linked to actor Nick Zano. In 2009, Cavallari appeared to be dating Justin Bobby, as seen on The Hills.