A judge on Thursday found an ex-NYPD cop guilty of assaulting a mentally ill homeless teen while he was handcuffed — then lying about it on police paperwork and to his supervisors.

Elijah Saladeen, 49, now faces up to four years in prison when he’s sentenced Nov. 12, but will remain free until then.

The convictions — which include misdemeanor assault, attempted assault and three counts of offering a false instrument for filing — will not affect his pension.

Manhattan Supreme Court Justice Robert Mandelbaum, however, acquitted Saladeen of two counts that related to allegedly lying to prosecutors about what happened during the arrest of Jeremy Santiago.

The judge ruled that “sloppiness or negligence in completing police paperwork is not a crime.”

As Saladeen exited the courtroom, he said, “I disagree with it [the verdict].”

He waived a jury trial, leaving the determination of his guilt in Mandelbaum’s hands.

The former housing cop responded to a complaint of a trespasser at the Fulton Houses Feb. 24, 2017.

He and his partner found Santiago, then 19, sleeping in the 25th-floor stairwell. After they roused him, Santiago, who suffers from bipolar and schizophrenia, became distraught and slammed his head against the wall. Saladeen and his partner, Natalie Roman, cuffed Santiago and escorted him to the lobby, where he began to struggle, wailing, “I don’t want to go to jail!” and spitting at them.

Saladeen wrestled him to the ground, then repeatedly punched him on the side of the head, causing a facial laceration that required stitches.

On the stand, the former cop said he acted in self-defense, only pummeling Santiago after his partner — who testified against him — yelled, “He’s about to bite you!”

But Mandelbaum said that silent surveillance footage, which was played in court, contradicted this claim, and showed that Saladeen slugged Santiago at least once before the attempted bite.

The judge added that even if the defendant tried to bite him before he threw the first punch, “The complainant was both handcuffed and lying face down on the ground, the defendant could have avoided any risk of injury to himself simply by pulling his hand away from the complainant’s mouth.”

Another camera angle shows Saladeen dragging Santiago, still cuffed behind his back, to a rear hallway and slugging him repeatedly in the ribs as his partner watches in dismay.

Mandelbaum said Saladeen had intentionally removed his prisoner to an isolated location and “gratuitously bestowed” the rib punches out of “malice.”

Defense lawyer Craig Hayes said they disagreed with the verdict and would appeal.

“Officers have a right to defend themselves just like any other civilian and you don’t have to wait for someone’s teeth to break the skin and draw blood,” the attorney said.

Saladeen was fired in September after an NYPD departmental hearing found him guilty of misconduct related to the incident.

“[Saladeen’s] criminal misconduct caused extensive injuries to his victim and damage to all in law enforcement who are working to strengthen our relationships with the communities we are sworn to protect,” DA Cy Vance Jr. said in a statement,