Montag was born September 15, 1986, in Crested Butte, Colorado, to parents Darlene and Bill Montag. After divorcing, their mother Darlene married Tim Egelhoff; they owned The Timberline restaurant for 21 years until its closing in 2010. Montag has an older sister Holly, also a reality television personality, and a younger brother Sky. Their father Bill later married Terri O'Hara; their step-brother Eric O'Hara died in 2008 after an accidental fall from an icy roof.

After moving to Los Angeles in 2006, the Laguna Beach spin-off series The Hills was developed to chronicle the lives of now-housemates Conrad and Montag and friends Audrina Patridge and Whitney Port. That year, she began a short-lived relationship with Jordan Eubanks; she described its end as "the best decision of [her] life". By the second season, Montag and Conrad's friendship had deteriorated after the former began dating and later moved in with Spencer Pratt. During the third season, Conrad ended her friendship with Montag after she suspected that Pratt was responsible for rumors of a sex tape involving her and her former boyfriend Jason Wahler; the ensuing feud carried through each subsequent season.

During the fourth season of The Hills, Montag and Pratt eloped to Mexico, where they married on November 20, 2008. A wedding ceremony was held April 25, 2009, in Pasadena, California. Their son Gunner Stone was born October 1, 2017.

After several additional unauthorized leaks, Montag enlisted Songwriter Cathy Dennis to continue work on her album. She released her first two extended plays Wherever I Am and Here She Is... in 2009. The former included the song "More Is More"; debuting at number 50 on the US Billboard Hot Dance Club Songs chart, it became her first and only song to chart in the country. Later that year, she and Pratt appeared on the second season of the American version of I'm a Celebrity...Get Me Out of Here! in support of the "Feed the Children" foundation. They quit after Montag was hospitalized with a gastric ulcer and later created controversy after alleging that they were subject to torture during production. After leaving the series, Montag and Pratt became notorious for their antics and antagonistic roles, notably during an interview with Al Roker of Today, and were described as "everything that's wrong with America". In August, she performed her first official single, the finished version of "Body Language", at the Miss Universe 2009; her appearance was met with a negative critical response, who criticized its overall production. The following month, she appeared on the cover of Playboy. In November, Montag and Pratt released the book "How to Be Famous: Our Guide to Looking the Part, Playing the Press, and Becoming a Tabloid Fixture".

After her exit from The Hills, Montag auditioned for the replacement of Megan Fox in the third installment of the Transformers series, but was ultimately not featured in the film. In June 2010, she and Producer Steve Morales commenced work on her unreleased second studio album. The following month, Montag filed for divorce from Pratt, citing irreconcilable differences in the petition. However, they called it off in September after confessing that the action was intended to boost Montag's ailing career. In November, they renewed their vows in Carpinteria, California. In February 2011, Montag made her feature film debut as Kimberly in Just Go with It. Later that year, she appeared on the VH1 reality television series Famous Food, where she competed against other celebrities for a partnership stake in a restaurant in which they worked to open. In October, Montag began writing a memoir. The following year, she released her third extended play Dreams Come True.

In January 2013, Montag and Pratt as a single entity competed as housemates on the eleventh series of the British version of Celebrity Big Brother, where they notably developed a minor feud with singer and television personality Rylan Clark. They were named the runners-up, losing to Clark. On February 18, Channel 5 aired a one-off television special discussing Montag and Pratt's rise to prominence, titled Speidi: Scandal, Secrets & Surgery!. Later that year, they launched the Speidi Show, which was initially assumed to be a web series in which the couple used a different reality television format for each installment. However, the project was later revealed to be an Example of networked improv narrative, where Montag and Pratt collaborated with Mark Marino and Rob Wittig to create a Twitter game in which players Live Tweet an imaginary show. In October 2013, Montag revealed that her original F-cup breast implants resulted in severe health issues, and underwent a breast reduction surgery to replace them with D-cup implants. In December 2013, Montag stated that she and Conrad have "talked a few times" since the series' conclusion, elaborating that "it's unfortunate things happened the way that it did, but we're both different people now, older and more mature." However, in 2015, Montag angrily excoriated Conrad when she appeared on Marriage Boot Camp, during a sequence where the contestants were asked to forgive someone who had hurt them in the past; Montag told a stand-in for Conrad that Conrad had "ruined my life with her lies", said that Conrad DID have a sex tape and had falsely blamed it on her, and that she "forgave" Conrad for being terrible and trying to ruin her life. Montag was universally lambasted for bringing up the sex tape lies anew, parroting the views of Pratt (who Conrad actually blamed for spreading the false rumors; Conrad felt Montag knew he was lying but took Pratt's side because she loved him) and trying to use Conrad's name to get publicity for herself. She and Pratt appeared in the television special After Shock: Heidi & Spencer, which premiered on December 9, 2013, on E!, during which they admitted that many of the situations they were involved with in the various reality series they starred on were in fact made up by the shows producers. Montag and Pratt later appeared on an episode of Celebrity Wife Swap in June 2014, for which they swapped with Olympic athlete Amanda Beard and her husband Sacha Brown. In October 2015, Montag and her husband made several media appearances that were largely devoted to two subjects: claiming that their image as terrible people stemming from The Hills was not factual and, somewhat divergently, their intense dislike of Lauren Conrad, which included Montag's husband admitting he spread rumors (which Montag said were true but have never been proven) that Conrad made a sex tape with a former boyfriend. Montag also claimed she had been paid to remain Conrad's friend during her time on the show, while also adding she had reconciled with her family to her husband's displeasure.

In 2017, Montag re-entered the Celebrity Big Brother with her husband. On day 18 the couple were eternally nominated by James Cosmo, which they did not have a good reaction to. They were saved in an 8-person eviction on day 22 however, they were evicted on day 25 against Kim Woodburn, Jessica Cunningham, Bianca Gascoigne and Jedward. Montag and Pratt were reunited with their rival, Rylan Clark-Neal, on Big Brother's Bit on the Side that night.