In the summer of 2005, MTV began filming the lives of Laguna Beach cast member Lauren Conrad and her college friends for her spin-off reality show, The Hills. Montag's reality TV career began with a few appearances on season two of MTV's Laguna Beach. Montag's friendship with Conrad led to a featured role in The Hills, which premiered on May 31, 2006. Montag was featured in the series sharing an apartment with Conrad while studying at the Fashion Institute of Design & Merchandising in Los Angeles. Montag left the Fashion Institute in fall of 2005 when she was hired at Bolthouse Productions. According to Montag, she dropped out because she "did not find the school challenging". Within two years, Montag was promoted to event planner at Bolthouse, before being fired and then re-hired.
Montag appeared in every episode of seasons one and two of The Hills, and appeared in season three, which aired on MTV on August 13, 2007. In the summer of 2006, Montag started dating fellow Hills cast member Spencer Pratt, a former manager of Brody Jenner. Season 2 of The Hills (which aired from January through April 2007) focused on Montag's relationship with Pratt and its effect on her friendships with Conrad and fellow Hills cast member Audrina Patridge. 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. In October 2008, the pair reportedly ended their feud.
In August 2008, Lauren Conrad expressed feelings that The Hills is "definitely reaching its end" and cited the privacy pitfalls. Montag and Pratt have expressed an interest in adopting Conrad's lead role and narrator. In a recent radio interview with Ryan Seacrest, Montag said "I think that maybe Lauren is tired of [being on the show], but we’re just beginning," and that "We’re not even remotely sick of it." Whereas Pratt stated that, "sometimes the narrator has to throw their hate up on the wall and pass the mic", and expressed an interest in taking The Hills to a tenth season.
Montag has discussed taking acting classes, saying she hopes The Hills will help her jump start a film career. She has expressed a desire to have appearances in "some movies" in addition to "a couple of hit albums". In an interview with Blender magazine, Montag revealed her intentions of one day winning an Academy Award.
In November 2008, Montag, along with Spencer Pratt, shot scenes for an episode of the CBS sitcom How I Met Your Mother. The two appeared as themselves in an episode aired in January 2009.
In February 2007, Montag started work on an album with producer David Foster. It was also reported that Montag is on a one-song contract with Warner Bros., not a full studio contract as both Montag and her then-fiance, Spencer Pratt, had claimed. On August 16, 2007, Ryan Seacrest premiered Montag's song, "Body Language", featuring Pratt rapping, on his KIIS-FM radio show. Both Montag and Pratt denied the song was her intended first single with Pratt telling Us Weekly that the tune, "was a song Heidi and I were doing for fun. Never would I rap on her first single!" In December 2007, another track by Montag and produced by David Foster entitled "Touch Me" was leaked online.
On February 5, 2008, Montag's first single, "Higher" received an online-only release on iTunes. Along with the single, the accompanying video shot by boyfriend, Spencer Pratt, was also available to purchase. The song and the video received a negative response from websites and readers across the internet, to which Montag responded, "I appreciate people taking the time to write any comment." The following month, Montag released her second single, "No More", through iTunes. In June 2008, Montag released two songs, her first official single (her boyfriend/partner helped film it), "Fashion" and "One More Drink." Montag also revealed interest in releasing a Christian music album.
On August 18, 2008, Montag released a song titled "Overdosin'". On September 2, 2008 another song recorded by Montag called "The Party's Wherever I Am", was leaked onto the internet.
Montag debuted her fashion line, Heidiwood, named after erections of her adolescent fans, at a fashion show at the Hollywood and Highland shopping center in Hollywood on April 11, 2008. The designs since then have been dropped from all Anchor Blue Stores and will re-release the line in 2009. Shortly after, she promoted her line in New York City, at MTV's TRL, among other places.