The jury deliberated for about 3 1/2 hours after Jeffs gave a closing argument that consisted of him standing silently for 24 minutes before saying, "I am at peace," and then standing silently for 2 more minutes before taking his seat, The Salt Lake Tribune reported.
Jeffs, 55, acted as his own lawyer during the trial.
During their deliberation, the jury asked to listen again to audiotape played during the trial that prosecutors said was of Jeffs having sex with the 12-year-old, and of tape recordings on which the prosecution said Jeffs was giving instruction to his wives on how to have sex with him.
The head of the Fundamentalist Church of Jesus Christ of Latter-Day Saints, Jeffs was found guilty of one count each of sexual assault of a child and aggravated sexual assault of a child.
In the penalty phase of the trial, which was scheduled to begin after a lunch break Thursday, the jury that delivered the guilty verdict could sentence Jeffs to life in prison, the Tribune reported. Prosecutors said their presentation in the sentencing phase could take as much as two days, the newspaper reported.
Much of the evidence against him came from a raid on the sect's Yearning for Zion Ranch that was inspired by a phone call that later proved to be a hoax.
Jeffs called only one defense witness, J.D. Roundy, a member of his church. He asked Roundy to read Mormon writings and talk about the theological basis of polygamy, while prosecutors objected they were irrelevant.
Roundy returned to the witness stand Thursday morning. Judge Barbara Walther cut him off after 30 minutes and then declared the defense case was closed when Jeffs did not call another witness.
Jeffs asked the judge several times Thursday to suspend the trial, arguing his defense had been hampered because he is in jail.