Alexia Douglas, 16, died after Christina Shiver, who had only just passed her driving test, lost control of her SUV in St. Petersburg, Florida, on Friday.
The car mounted the pavement and smashed into a barrier. Metal guard rails pierced through the passenger's side windshield, killing Alexia immediately.
Christina, also 16, was treated in hospital but only suffered minor injuries. A witness said that she believed her best friend was still alive and wanted to ride with her in the ambulance to hospital.
Alexia's mother Barbara Nicholl paid tribute to the girls' friendship.
"They were always, always together," she said. "They got along with everybody, but they always said they were each others' only friend. Every weekend it was Alexia and Christina."
On the night of the tragedy, the girls did not have any set plans, but discussed going to the mall or watching films together.
Police said Christina had been driving the girls in her 2008 Ford Escape when she became distracted and swerved right, mounted a barrier and crashed into a guard rail.
An investigation is ongoing.