A beloved Nassau County crossing guard who was rammed into by an accused drugged driver has died — and the motorist was allegedly high on “street Xanax” at the time, prosecutors revealed Tuesday.

Joshua Alvarado, 30, now faces upgraded charges of manslaughter for allegedly jumping a curb and mowing down 70-year-old John Miro as he was on-duty helping children get to school in Merrick, the Nassau County District Attorney’s Office said.

Miro succumbed to his injuries on March 6, a week after the crash, prosecutors said.

Alvarado, of Queens, was initially arrested by police on assault charges, but he now faces an indictment that includes manslaughter following Miro’s death, according to prosecutors.

The motorist allegedly had “street Xanax” — an illegal and highly potent synthetic drug — in his system at the time of the Feb. 26 crash, the district attorney’s office said.

Alvarado was behind the wheel of a commercial pickup truck on the way to a service call for an extermination business when he fell asleep at a red light at the intersection of Sunrise Highway and Merrick Avenue around 8 a.m., prosecutors said.

When loud honks woke him up, he zipped diagonally past the intersection, jumping the curb and hitting Miro, who was on the job, clearing a sidewalk of snow, authorities said.

Miro suffered blunt force trauma to his head, a broken hip and numerous shattered ribs.

Toxicology reports indicated Alvarado was also allegedly on the anxiety drug Xanax, as well as Clonazepam, which can be used to treat certain seizure or panic disorders, according to the Mayo Clinic.

“A beloved crossing guard is dead because this defendant allegedly drove while highly impaired by a powerful mixture of sedatives and illegal street drugs,” said District Attorney Anne Donnelly.

“He allegedly struck down John Miro on the very sidewalk where he had helped children safely cross for years, leaving a permanent void in the lives of everyone who knew him,” Donnelly said in a statement.

“We intend to seek justice for Miro’s family and the community that relied on his consistent, reassuring presence.”

Alvarado is facing manslaughter, vehicular manslaughter and assault charges, all in the second degree, and driving while impaired by drugs counts.

He pleaded not guilty and was remanded behind bars pending his next court date. If convicted, he could face between seven and 15 years in prison.

A call to his lawyer was not immediately returned.

The union representing crossing guards, CSEA Nassau Local 830, said earlier this month Miro “served with quiet courage and dedication.”

A fundraiser in his honor stated he performed his job “with a smile and a kind word.”