Mr. Simcox, 55, was found guilty last month of molesting the girl, who was a friend of his 6-year-old daughter, in 2012 and 2013 during scheduled parental visits at his apartment after he and his wife divorced.

The 5-year-old girl told her mother of the abuse, who called law enforcement.

In a statement on Monday, the Maricopa County attorney, Bill Montgomery, said the 19-and-a-half-year sentence involved two counts of molestation of a child and one count of furnishing obscene materials to a minor.

“The defendant committed unconscionable acts against a young child, the emotional and psychological effects of which she’ll have to deal with her entire life,” Mr. Montgomery said. He added that the sentence “won’t take back the harm he has done, but it removes the opportunity for him to prey on another child.”

During the trial, Mr. Simcox was also accused of sexual conduct with another young girl but was found not guilty of that charge.

Mr. Simcox’s trial took several unexpected turns. Although he is not a lawyer, Mr. Simcox represented himself and sought to question the girls he was accused of molesting.

Prosecutors brought in an expert to address the trauma it could cause the girls, and they took action in lower courts to block Mr. Simcox, eventually filing a petition that is now before the United States Supreme Court and could have an impact on future case law.

Mr. Simcox allowed his advisory attorney, Kerrie Droban, to question the girls instead. Ms. Droban said in an emailed statement on Tuesday that Mr. Simcox planned to appeal the sentence, which amounts to 17 years for each molestation charge, to run concurrently, and 2-and-a-half-years for the obscene materials charge. With time already served of about 3 years, he would have less than 17 years remaining, she said.

Mr. Simcox was a high-profile advocate of tougher control of borders and undocumented day laborers. He was a founder of a volunteer armed watch group, Civil Homeland Defense, which was created after the Sept. 11 terrorist attacks, and which intercepted undocumented immigrants crossing into Arizona, turning them over to the authorities.

Later, he helped form the Minuteman Civil Defense Corps, which stepped into the national spotlight in 2005 with its border patrols. In 2012, Mr. Simcox told the Southern Poverty Law Center, which was investigating the possible involvement of extremist groups in the killings of Mexicans crossing the Arizona border, that he had left the movement and was living a quiet life tutoring schoolchildren in suburban Phoenix.

The victim’s mother, speaking to reporters after the hearing, said her daughter had not spoken much about the incidents but had great support from her family, according to the Arizona Republic. She said she had recently asked her daughter if she wanted to make a victim impact statement for the sentencing.

“The only thing she said was, ‘He messed with the wrong family,’” the mother said.

When asked for her opinion of Mr. Simcox, the woman said she viewed him “in the same way I would view anybody who did what he did to my child — I’m glad he’s gone.”