State Park Police said they first became aware of the stranded child at the beach near Field 6 about 3:30 p.m. and began searching for the girl's parents.
No one there claimed the child, police said, so other law enforcement departments and social services authorities were alerted.
About 6:30 p.m., someone from Our Little Darlings Day Care in Bay Shore called park police to ask whether anyone had found a little girl, police said.
The child's mother was then located and reunited with her unharmed daughter Tuesday night, authorities said.
"The cop there told me the mother's so distraught that they're going to drive her home," said State Park Police Capt. Bruce Marx.
Marx said a woman at the beach first notified lifeguards about the child left behind.
"This kid was playing with her kid for about an hour," Marx said. When the woman wanted to leave, he said, "she started wondering, 'Where's this child's mother?' "
Marx said the investigation is continuing and no charges have been filed.
No one could be reached at the day care center for comment.