In the summer of 2005, MTV began filming the lives of Laguna Beach cast member Lauren Conradand 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 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 has no plans to return to fashion school.

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.

The nature of Montag's role in future seasons of The Hills is unclear since she is no longer living with Conrad and no longer speaks with her because of conflicts which occurred during their time as roommates, which surrounded Montag's relationship with Spencer Pratt 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 Patridgemade 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". In a recent Blender magazine interview, Montag revealed her intentions of one day winning an Academy Award.

As of February 2007, Montag is working on recording an album with producer David Foster. 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."