Wanda Eileen Barzee apologized in court, saying she was "humbled as I realize how much Elizabeth Smart has been victimized and the role that I played in it."
"I am so sorry, Elizabeth, for all the pain and suffering I have caused you and your family," Barzee, 64, said. "It is my hope that you will be able to find it in your heart to forgive me."
Smart, now 22 and preparing to serve a mission in Paris for The Church of Jesus Christ of Latter-day Saints, was not in court. But her father, Ed Smart, made a statement there.
"I just hope that Wanda realizes what she did and that it was absolutely wrong and absolutely horrible," he said.
Barzee, who wore a black skirt and black flats, pleaded guilty to charges of kidnapping and unlawful transportation of a minor in U.S. District Court in Salt Lake City.
Sentencing for Barzee was set for May 19.
She could have faced a life sentence for the kidnapping charge and up to 15 years for the other count. However, under the plea deal, she is expected to receive 15 years in prison, with credit for about six years already served.
Barzee will be returned to the Utah State Hospital until her sentencing. She also will plead guilty in state court under the plea deal to one count of conspiracy to commit aggravated kidnapping and will cooperate in the case against her estranged husband, Brian David Mitchell.
Smart was 14 when she was taken at knifepoint from the bedroom of her Salt Lake City home, sparking a search that riveted the nation. Nine months later, in March 2003, Barzee and Mitchell were arrested after they were spotted walking on a suburban street with Smart.
Barzee's role in the abduction has garnered less attention than Mitchell's.
At a hearing last month, Smart said that within hours of the abduction, Mitchell took her as a polygamous wife and then raped her. Smart said Barzee washed the teen's feet and dressed her in robes before the ceremony.
Barzee often became upset over Mitchell's relationship with Smart, but that sentiment would never last, Smart said.
A 10-day competency hearing in Mitchell's case is set to begin Nov. 30 in federal court. Mitchell, 55, and Barzee were indicted on federal charges in March 2008.
Mitchell, a one-time itinerant street preacher, is accused of taking Smart as a wife in order to fulfill a religious prophecy included in a 27-page manifesto he wrote called "The Book of Immanuel David Isaiah."
Smart said she was raped daily by Mitchell throughout her nine months of captivity. She said he threatened to harm her if she ever tried to escape.