Wanda Eileen Barzee, 63, was indicted on charges of kidnapping and unlawful transportation of a minor.
Attorney Scott Williams said she will plead guilty to both charges Tuesday in U.S. District Court in Salt Lake City. A final agreement was reached Monday, he told The Associated Press.
Barzee and her estranged husband, Brian David Mitchell, were both indicted in March 2008, six years after Smart was taken from her Salt Lake City home at knifepoint. She was found nine months later walking with the couple a suburban street.
Melodie Rydalch, spokeswoman for the U.S. attorney's office, said Monday she could not comment beyond what was contained in papers filed earlier in the day in U.S. District Court in Salt Lake City.
A decorum order for the expected arraignment Tuesday issued by U.S. District Judge Dale Kimball said Barzee "will be arraigned and is anticipated to enter into a guilty plea pursuant to an agreement with the United States."
Previous state court cases for the pair were stalled after both were ruled incompetent for trial. Barzee was ordered to under go forced medication treatments in 2007.
Last month, a state hospital report to a state judge said Barzee was now competent.
"No issues of competency will be raised in the federal court matter," Williams said Monday.
Mitchell's case is also moving through federal court. The 55-year-old onetime itinerant street preacher is scheduled for a 10-day competency hearing beginning Nov. 30.