"We had a beautiful ceremony here ... officiated by a minister and photographed by the hotel photographer. We've never been happier," said the couple in a statement provided by Us Weekly. "And, like other elopements that happen outside the country, we'll take care of the legal details when we get home."
They did not say specifically when where they would seek to make their marriage official, which would likely require a separate ceremony in the U.S.
The public pair provided photos to the magazine for its issue on newsstands Wednesday showing their ceremony at a chapel near Cabo San Lucas, Mexico on Nov. 20. The accompanying story features an interview with Montag and Pratt, as well as details about the rings, Montag's dress, and the content of their handwritten vows.
The story doesn't note that they had neither obtained a marriage license nor taken part in a separate civil ceremony, which is required by Mexican law to make the union binding.
A couple can register their marriage up to 10 days after a ceremony, but California does not recognize marriage ceremonies outside the United States, according to the state's Department of Public Health. A couple can petition a court later to have their marriage legally recognized.
Us Weekly's report, which was picked up by numerous celebrity magazines and blogs, said that none of the couple's friends or relatives was in attendance. Some people close to the couple said they were caught off-guard.
"Everybody's pretty upset," Montag's sister, Holly Montag, told MTV on Monday. Montag's father, Bill Montag, told People magazine that he'd be dismayed if his daughter got married and didn't invite him.
If the story were true, he added, "Then we just have to have another wedding here!"
Us Weekly, which has featured the couple in numerous cover stories and photo spreads, reported that the two decided to get married while drinking margaritas on the beach. An hour later, they were exchanging vows, the magazine said.
Americans who wish to get married in Mexico must go through a process that takes about five days, according to Mexico's foreign relations department Web site. They must obtain a health certificate, including blood test results from a local doctor; and provide official translations of legal documents, such as birth certificates.
Only civil, nonreligious marriages conducted in Mexico are valid internationally. Peter Grossman of Us Weekly, who conducted the wedding-day interview with Pratt, 25, and Montag, 22, confirmed that the two exchanged rings and handwritten vows in front of a minister.
"Heidi and Spencer are two people in love who had a beautiful wedding ceremony to celebrate that," Grossman said. "The pictures and words in our issue this week bear that out."
"The Hills" executive producer Liz Gateley said Wednesday that the couple's wedding and honeymoon will appear in future episodes of the MTV reality show.
"MTV is currently filming Heidi and Spencer in Mexico, as we were expecting to capture them on vacation," Gateley told MTV News. "Fans will be able to see the exclusive footage from the ceremony in an upcoming episode, and we will continue to follow the newlyweds' relationship as it plays out in real life."
Since Montag and Pratt's pairing came to light on the MTV reality show, the two have been ever camera-ready, willing to pose or invite photographers to capture their "private" moments - including an intimate, champagne-and-roses Valentine's Day celebration aboard a chartered boat earlier this year.