“It took tremendous courage for this young woman to share her story at trial,” Erin Nealy Cox, the United States attorney for the Northern District of Texas, said in a statement on Monday. “She was brought to this country at a young age, pressured to stay quiet, and forced to work for this family without pay for 16 years. I want to commend her, as well as the witnesses who helped shine a light on her circumstances.”

The authorities said that in 2000, Mr. Toure and Ms. Cros-Toure arranged for the girl, then 5, to travel alone from West Africa to their Southlake home on a tourist visa.

After arriving in Southlake, she would start working by 7 a.m. every day, cleaning, making beds, vacuuming, cooking and gardening, among other chores, she told the authorities. They said she was forced to take care of the couple’s children.

The authorities said the young woman was physically abused and struck at least once with an electrical cord. She told investigators that she had visited a doctor only once and had slept on a floor for years, upgrading to a twin bed only when one of the couple’s children left for college.

The young woman was isolated from her family and prevented from receiving any education while the couple’s children went to school and college, the authorities said.