After graduating from high school in 2004, Montag moved to California to study at the Academy of Art University in San Francisco. At freshman orientation, she met Lauren Conrad, star of MTV’s hit reality show, “Laguna Beach” and the two became fast-friends.
Shortly after becoming friends, Montag and Conrad decided to move to L.A. to study at the Fashion Institute of Design and Merchandising.
In 2006 MTV created a “Laguna Beach” spinoff featuring Conrad called “The Hills,” which chronicled Conrad's move to L.A. as she studied fashion, interned at Teen Vogue magazine and lived with Montag. In the first season, Montag dropped out of FIDM to work for the event-planning firm, Bolthouse Productions, in which she is first quickly promoted but later fired.
During the second season of “The Hills” in 2007, Montag and Conrad’s friendship began deteriorating when Montag began dating bad-boy and fellow “Hills” cast member Spencer Pratt, a former manager of Brody Jenner, Conrad’s ex-boyfriend.
Throughout Season 2, Montag's relationship with Pratt and its effect on her friendships with Conrad and fellow Hills cast member Audrina Patridge are featured. On the Season 2 finale, Montag moved out of her apartment with Conrad to live with Pratt. Although Conrad maintained the two were still friends after Montag moved out, the friendship soon ended when Conrad blamed Montag and Pratt for spreading rumors about a sex tape featuring her and ex-boyfriend Jason Wahler.
Later in 2007 Montag got a nose job and breast augmentation, taking her bust size from an A-cup to a voluptuous 32C. "It was really something that had been tormenting me and I was very insecure about it," People magazine reported of her pre-surgery looks. "I thought about it for several years and I waited until I had the finances and really thought about it and was old enough to really make a mature decision."
In an interview with Blender magazine, Montag told Blender magazine her life aspirations include winning an Academy Award among other lofty goals. In January 2009 Montag and Pratt appeared as themselves on the CBS sitcom “How I Met Your Mother.”
In the third season premiere of “The Hills,” Conrad screams at Montag, "You know what you did!" And so the cat-fights continue; many TV critics question the authenticity of the show, the tiffs between Conrad and Montag drive 3.6 million viewers, the highest rated show for MTV in 2008.
Later in the third season Pratt tells Entertainment Weekly, "We would get engaged; we just wouldn't do it on Lauren's show," and yet Pratt and Montag do just that. Around that time, Montag appeared on the now defunct teen magazine CosmoGIRL!, from the publishers of Cosmopolitan magazine. Readers of CosmoGirl! question editors' judgment, calling Heidi "plastic and fake" and say she "trashed her best friend."
By 2008 Heidi appears on the cover of the men’s magazine Maxim magazine, in which she says, "[Lauren's] changed. Some people go more Hollywood than others...I’ve heard from other people that she’s a big diva now."
Attempting to breakthrough as a singer, Montag releases her music video, "Higher," which received an almost unanimously negative reception by People magazine readers. In April 2008 Montag launched her fashion line, Heidiwood at the Hollywood and Highland Center; during this time Montag endorsed presidential candidate John McCain in an issue of Us Weekly. McCain responded to Montag’s endorsement in Time magazine, "I'm honored to have Heidi's support and I want to assure her that I never miss an episode of “The Hills,” especially since the new season started."
Surprisingly, also in April, Montag and Conrad appeared together – with Audrina Patridge, and Whitney Port, – on the cover of Rolling Stone magazine. The shoot is the first time the two were photographed together since their ratings-boosting feud. In October 2008, the pair reportedly ended their feud.
While on vacation in Cabo San Lucas, Mexico, Montag and Pratt eloped on November 19, 2008, reported Us Weekly magazine. While Montag told Us Weekly magazine, “The minute we said our vows, I couldn’t stop crying,” her parents told People magazine of their disapproval, "I would be upset if she got married and didn't invite me!" Montag’s father Bill told People magazine. Montag's mother, Darlene Egelhoff, also voiced her disapproval, even claiming Pratt "drugged" Montag into marrying him. The wedding was filmed for season four of “The Hills,” but remains to be made legal, as California does not recognize marriages performed outside of the U.S.
