Nassau jurors convicted a motorist Friday of manslaughter in a parkway crash that instantly killed a Hempstead man on his way to work, rejecting the defense’s claim that the accused wasn’t behind the wheel during the December 2014 wreck.

Jurors found Madi Grant, 35, of Oyster Bay, was drunk and high on marijuana as he sped east on the Southern State Parkway and struck the back of Sherman Richardson’s car.

The impact about 6:30 a.m. on Dec. 5, 2014, propelled Richardson’s car into a tree on the eastbound roadside near Farmingdale. The 59-year-old union ironworker had been preparing for retirement at the time of his death.

Richardson’s widow, Jawana, began crying as she heard the guilty verdict and turned and hugged State Police Investigator Charles Fontanelli, the case’s lead investigator.

“Justice for Sherman,” she said later with a smile. “I am so excited the jury got it right. . . . This is the closure I needed.”

Prosecutor Stefanie Palma said the verdict was noteworthy because even though Grant fled the scene, authorities found witnesses and were able to prove Grant was intoxicated at the time of the crash.

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The verdict included convictions for vehicular manslaughter, leaving a deadly crash and an arson charge — after Grant tried to destroy evidence by arranging for the vehicle he had been driving to be set on fire.

Richardson’s widow embraced Nassau District Attorney Madeline Singas after the verdict, saying she would join prosecutors in their fight to change the law to enhance penalties for drivers who flee fatal crashes.

“The laws have to change so that people aren’t incentivized to run from scenes like this, leaving behind people who are dying and wrecking families,” Singas said.

Authorities said Grant fled the crash in the borrowed, rented Chevrolet Captiva he was driving, exited the parkway, and dodged a concerned driver who tried to follow him through the side streets of Amityville.

That witness gave 911 a partial license plate, and authorities found the SUV on fire hours later by the Copiague home of Grant’s girlfriend.

Grant is facing up to 15 years in prison on the manslaughter and arson convictions, and prosecutors said he could face consecutive sentences.

During the trial, defense attorney Donald Rollock pinned the crime on the government’s key witness — convicted drug dealer Joseph Parks, 37 — saying he was the driver and Grant was the passenger.

Rollock said after the verdict that Grant would file an appeal.