A mentally ill man en route to a Long Island hospital unfastened himself from a stretcher, opened the back door of an ambulance, jumped out and was fatally struck by another vehicle, whose driver fled the scene, cops said.

Frank Ligrnetta, 51, of Bay Shore, was being driven Monday evening from Stony Brook University Hospital to Brunswick Hospital Center, a psychiatric facility in Amityville, according to Suffolk County police.

He was sitting on the stretcher and was properly strapped in the rear, where an attendant sat in a jump seat, cops said.

A dark-colored sedan also traveling south on busy Nicolls Road in Holtsville at about 7 p.m. hit Ligrnetta, who was declared dead at the scene.

“This individual exited the vehicle as the ambulance was moving,” said acting Police Commissioner Tim Sini. “He hit the ground. He was never standing, and he was run over shortly thereafter.”

It was unclear why Ligrnetta jumped out.

“At this point, there does not appear to be a struggle,” Sini said.

The nature of Ligrnetta’s mental illness wasn’t released. He lived alone.

Sini said it was unlikely that the sedan’s driver was unaware that he had struck the man, Newsday reported.

Cops did not say how fast the private ambulance was traveling at the time of the incident.

Hunter EMS, whose ambulance was impounded for a safety check after the incident, did not return a call for comment.

Anyone with information is asked to call the Vehicular Crime Unit at 631-852-6555 or call Crime Stoppers anonymously at 800-220-8477.