The first of a dozen polygamist sect members to be tried over charges including child sex offences faces jail after being found guilty.
Raymond Jessop, 38, was convicted in a Texas court of sexually assaulting an underage girl with whom he had entered a so-called spiritual marriage.
The Fundamentalist Church of Jesus Christ of Latter Day Saints' member could now be jailed for up to 20 years when he is sentenced on Monday.
He was among 12 people charged following a 2008 raid at the Yearning For Zion Ranch in Eldorado, Texas, during which more than 400 children were removed - making it the largest child-custody case in US history.
The compound houses a sect that believes polygamy brings glorification in heaven.
Jessop is said to have nine wives, including a daughter of sect leader Warren Jeffs. Prosecutors alleged that he married the girl a day after her 15th birthday.
The girl in Jessop's assault case was previously in a spiritual marriage with his brother before being "reassigned" at the age of 15, according to documents seized at the ranch.
She became pregnant aged 16 and gave birth to a child after several days in labour in 2005.
Sect leaders refused to take her to hospital out of fear that authorities would discover her age.
In a journal taken by federal agents, Jeffs wrote: "I knew that the girl being 16 years old, if she went to the hospital, they could put Raymond Jessop in jeopardy of prosecution as the government is looking for any reason to come against us there."
Defence lawyers argued that no witness could place the accused man at the ranch at the time the girl fell pregnant.
But prosecutors used photo albums, family records and writings by Jeffs to establish a time line that put Jessop at the ranch at the time of the assault.
A forensic expert said there was a nearly 100% probability that he was the father of the child, now aged four.
Jurors needed little more than two hours to return a guilty verdict.
Jessop did not react as the decision was read out. He was led to the cells in handcuffs ahead of sentencing on Monday.
Bigamy charges against the convicted paedophile will be dealt with at a later date.
Jeffs is currently behind bars awaiting a trial in Arizona relating to underage marriages there. He will then face sexual assault and bigamy charges in Texas.
The sect leader has previously been convicted as an accomplice to rape for his part in arranging an underage marriage in Utah.Reuse content