The 19-year-old victim was riding a scooter with her sister and a friend on Bellport Lane in Bellport around 9 p.m. Tuesday.
Police say Helen Geraghty struck the girl with her 1999 Volkswagen Passat.
The girl was airlifted to Stony Brook University Hospital, where she was being treated.
She is expected to recover.
Geraghty is due in court to be arraigned on drunk driving charges.