Harman was originally charged with first-degree felony rape, but was allowed to plead to the lesser crime in exchange for testifying against Terrill Dalton, the president of the Church of the Firstborn of the General Assembly of Heaven.
Dalton, 45, was convicted by a jury last month of two first-degree felony counts of rape and faces up to life in prison when he is sentenced May 11.
The crimes occurred in 2005 and 2006, when the church was based in Magna.
The sect, which has never had more than 100 members, left Utah and moved to Idaho before settling in southern Montana.
At trial last month, the victim, now 22, said Dalton told her that having sex with Harman was God's will. The woman said Dalton later promised her "great blessings" if she also had sex with him.
"He made me feel as if no matter what I did, I could never be right before God," she testified. "I tried so hard. ... There was always something bigger he wanted me to do."
Dalton's defense attorney, Rudy Bautista, had argued that the woman waited years to report the rapes and had been inconsistent with her story ever since.
"When someone's story keeps changing, there's a reason," Bautista told the jury. "They can't remember the lie they told."
But Salt Lake County prosecutor Tupakk Renteria called those arguments "red herrings."
"She was raped at 15 years old by a person who should have protected her," Renteria said at trial.
Dalton told the girl he was "the Holy Ghost" and church members called him "God in the flesh," officials said. The prosecutor said Dalton "served her up on a platter" to Harman, who was the church's second in command.