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.