The surprise plea agreement with prosecutors came just before closing arguments were set to begin in the trial of Jordan Graham, 22.
In exchange for the guilty plea to second-degree murder, prosecutors agreed to drop a first-degree murder charge and a count of making a false statement to authorities.
Graham could face a maximum sentence of life in prison when she is sentenced on March 27.
In accepting the plea, U.S. District Judge Donald Molloy told Graham to recount exactly what happened the night of July 7 when her husband Cody Johnson, 25, fell to his death in the park.
Graham said she decided to confront Johnson about her doubts and she did not know how he would take it. They climbed down a treacherous slope below a popular spot in the park called The Loop and spoke on a narrow ledge hundreds of feet above a ravine.
She told Johnson that she wasn't happy and wasn't feeling like she should after getting married. They argued, and at one point, she said, he grabbed her by the arm and she thought he was going to pull her.
She told the judge she got angry at Johnson, brushed his hand away then pushed him, with one hand on his arm and one on his back.
"I wasn't thinking about where we were. ... I just pushed," Graham told the judge.
She said she then drove back to her home in Kalispell without calling for help because she was so afraid she did not know what to do.
Graham's mother, Lindy Rutledge, buried her head in her husband's jacket sleeve as the judge questioned her daughter.
U.S. marshals then led away Graham in handcuffs.
Afterward, federal public defender Michael Donahoe said prosecutors had proposed the plea agreement.
It was Graham's choice to accept it, he said.
Prosecutors referred questions to U.S. Attorney Mike Cotter, who declined to comment before Graham's sentencing.
Johnson was reported missing on July 8 when he failed to show up for work.
Graham initially told investigators that Johnson left their house late July 7 with unidentified friends in a dark-colored car with Washington state license plates. Friends of Johnson testified they were suspicious of the story and suspected Graham played a role in his disappearance.
Graham showed police a fabricated email purportedly from a friend of Johnson that said Johnson was dead and to call off the search. The next day, she told Glacier park rangers she found Johnson's body near The Loop because it was a place he wanted to see before he died.
She finally acknowledged that she was with Johnson on the cliff after investigators confronted her with a security camera photo of the couple entering the park.
Prosecutors presented jurors with dozens of text messages between Graham and her friend from church, Kimberly Martinez, that documented how Graham's nervous excitement at the prospect of the wedding turned into despair over the week that followed.
Graham's attorneys said her doubts about her marriage did not mean that she would intentionally harm him.
They attempted to chip away at the prosecution's image of Graham as a cold, dispassionate woman who didn't want to marry Johnson, and their contention that she led him to a dangerous precipice in the Montana park and deliberately pushed him to his death.
They showed the jurors pictures and videos of Graham smiling as she had her hair done and tried on her borrowed wedding dress, then videos of the June 29 wedding and the couple's first dance.
Copyright 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.