The girl, who was abducted in July 1998, was found after a tip.
Gloria Williams, 51, have been charged with kidnapping, authorities in Walterboro, South Caroline, have said.
Kamiyah Mobley was living under another name and believed Gloria Williams to be her mother. Her biological family has been notified of the news.
Jacksonville Sherriff’s Office said DNA tests had confirmed Kamiyah Mobley’s identity.
She appeared in good health, a “normal 18-year-old woman”, the office said.
It said it had acted on 2,500 tips since the abduction and received one last year to @MissingKids that eventually broke the case.
The office showed the original composite images released at the time of the kidnapping, along with a photograph of Gloria Williams, who was arrested at her home on January 13.
The baby girl was only eight hours old when she was taken by a woman posing as a health care worker at the University Medical Center, now known as UF Health Jacksonville.
Gloria Williams told Kamiyah Mobley’s mother, Shanara, that her baby had a fever and needed to be checked. The woman took the baby out of the room and disappeared.
The case brought significant media attention as Shanara Mobley tried to locate her child.
Although the biological family had been notified of the discovery and were “elated”, the Jacksonville Sherriff’s Office said: “It is up to the victim on how contact will be made. The victim is now an adult.”
Jacksonville Sheriff Mike Williams said: “She had an idea that something may have occurred but… imagine her trying to process this.
“She was abducted as a newborn and needs time to process this… We want to respect her privacy and we ask that you do too.”
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