In the summer of 2005, MTV began filming the lives of Laguna Beach's Lauren Conrad's college friends for her spin-off reality show, The Hills. Montag was featured in the series sharing an apartment with Conrad while studying at the Fashion Institute of Design & Merchandising in L.A.. Montag left the Fashion Institute in fall of 2005 when she was hired as an intern at Bolthouse Productions. According to Montag, she dropped out because she "did not find the school challenging." Within two years, Montag was promoted from intern to event planner at Bolthouse, and she has no plans to return to fashion school. Her promotion left many MTV viewers wondering if it was a stunt for the cameras, as another of Montag's coworkers, Elodie, seemed much more qualified for the job and had been at Bolthouse longer. Elodie ended up leaving Bolthouse over the incident.

Montag's reality TV career began with a few appearances on season two of MTV's Laguna Beach. Her friendship with Conrad led to a featured role in The Hills, which premiered on May 31, 2006. Montag appeared in every episode of seasons one and two of the show, and appeared in season three, which aired on MTV on August 13, 2007.

The nature of Montag's role in future seasons of The Hills is unclear since she is no longer living with Conrad. Despite a major role on the show, she did not appear on The Hills: Finale After Party, broadcast live from MTV's New York headquarters or The Hills: MTV Canada Finale After Show (filmed at MTV's Toronto studios).

At The Hills: MTV Canada Finale After Show, cast member Audrina Patridge made it public that she no longer speaks with or sees Montag.

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".

As of February 2007, Montag is working on recording an album with producer David Foster (Brody Jenner's stepfather). She describes the sound as "very fun, fresh, sexy" Montag danced as a child, thus reportedly plans to do "a lot of performing" in connection with the album. Additionally, she indicates that a lot of her songs "will be very sexual."

It was also reported that Montag is on a one-song contract with Warner Bros. and not a full studio contract, as both Montag and her fiance, Spencer Pratt, had claimed.

On August 16, 2007, Ryan Seacrest premiered Montag's song, "Body Language", featuring Pratt rapping, on on his KISS-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!"

She recently filmed a music video for her debut single "Higher," featuring fiance Spencer Pratt as the director and producer.

Montag began dating Jordan Eubanks in 2005 and their relationship was featured on season one of The Hills. Montag broke off her relationship with Eubanks in the spring of 2006, a move that she has described as "the best decision of [her] life" and "something that [she] should have done earlier".

In the summer of 2006, Montag started dating fellow Hills cast member, Pratt, a former manager of Brody Jenner, met Montag through Jenner's then-girlfriend, Laguna Beach's Kristin Cavallari. Season two of The Hills (which aired from January through April of 2007) focused on Montag's relationship with Pratt and its effect on her friendships with Conrad and Patridge. On The Hills' season two finale, Montag moved out of her apartment with Conrad to live with Pratt.

In May 2007, Montag and Pratt announced their engagement. Fans of The Hills have speculated that the engagement is fake, a claim that both Montag and Pratt deny.