The 2-year-old was last seen wearing a black shirt and skirt with floral design and black sandals.
"Johnson was abducted by Sharena Nancy, 25 who has black hair and brown eyes," state police said.
Family members told WPXI that Nancy was a jitney driver who had given a ride to Nalani, her father, and a friend. When Nalani's father got out of the vehicle, Nancy allegedly sped away with the child.
"She was in a car seat in the back and he went around to get the car seat out with his daughter. And at the time he had his hand on the door and she drove off," Nalani's grandfather, Paul Johnson, told WPXI.
Pennsylvania State Police said Nancy was driving a 2017 black Toyota Prius with PA plate KLW3926 with an Uber sticker on the front passenger side window.
That vehicle was later located Saturday night on Rodi Road in the Penn Hills area with Nancy inside. The 25-year-old is now being questioned by police and no charges have been filed, WTAE reported.
Nalani, however, has not yet been found, according to police.
The FBI's Missing Children's Task Force is now assisting in the investigation, KDKA reported.
Anyone with information regarding Nalani Johnson or Sharena Nancy should immediately contact the police by calling 911 or the Penn Hills Borough Police Department at 412-473-3705.