Kamiyah Mobley learned her true identity on Friday and was reunited with her birth family by video chat.
Ms Mobley, who was living in the small town of Walterboro, South Carolina, under the name Alexis Manigo, defended Gloria Williams on Facebook.
She wrote of Williams, who has been arrested on a kidnapping charge, that she "raised me with everything I needed and most of all everything I wanted. My mother is no felon.
"The ignorant ones won't understand that."Video: Facebook / @JacksonvilleSheriffsOffice
In Jacksonville, Florida, the victim's family cried "tears of joy" after a detective told them their "baby" had been found.
"She looks just like her daddy," her paternal grandmother, Velma Aiken, told The Associated Press.
"She acts like she been talking to us all the time. She told us she'd be here soon to see us."
Joseph Jenkins, a neighbour of Williams in Walterboro, told the AP that Ms Mobley "wasn't an abused child or a child who got in trouble. But she grew up with a lie for 18 years."
Ms Mobley was only eight-hours-old when she was taken from her young mother, Shanara, by a woman dressed as a nurse at Jacksonville University Medical Center who told her the infant needed medical attention. The phony nurse fled with the baby.
A massive search by police on the ground and in helicopters was fruitless. But despite thousands of tips through the years, Mobley was never found. Some months ago, the young woman "had an inclination" that she may have been kidnapped, the Jacksonville sheriff said.
Authorities didn't say why she suspected this or how her case came to the attention of the National Center for Missing and Exploited Children.
But the centre contacted cold-case detectives, and Mobley provided a swab of her cheek for DNA analysis that proved the match, authorities said.
The birth family never forgot the infant taken from her mother's arms that day in July 1998.
"It's stressful to wake up every day, knowing that your child is out there and you have no way to reach her or talk to her," the girl's mother, Shanara Mobley, said in 2008.
Kamiyah was allowed to spend a few moments Friday with Williams, who will be extradited to Florida.
"She cried 'Momma' through the window of a security door," News4Jax reported.