When Gideon Moncrieffe looks in the mirror, he’ll always be reminded of the price he paid for being a Good Samaritan.

An eight-inch long gash runs from the top of his head to the bottom of his left cheek, the work of an unhinged ex-con who viciously attacked him after he tried to defuse a violent altercation last month on a Brooklyn train — an intervention fueled by the chokehold death of Jordan Neely 10 days earlier.

“When I saw that all I thought about was Jordan Neely and I thought I just didn’t want this to escalate,” he told The Post.

“I didn’t want someone else to get killed or hurt.”

The mayhem unfolded May 10 on the southbound C train in Brooklyn where Moncrieffe, 44, the global head of event production at TikTok, sat on his way back home to Stuyvesant Heights after grabbing dinner with a friend and noticed Sean Lewis, 33, squabbling with an older drunk man, who fell to the floor during the spat, although Moncrieffe said he didn’t know if the man was pushed or toppled over when the train lurched.

Moncrieffe said he asked whether Lewis was OK, hoping to prevent the situation from escalating, but Lewis responded by shoving him.

“I said look, somebody was killed on the train two weeks ago, they were choked out because someone proceeded to be aggressive,” Moncrieffe said, referring to ex-con Neely, who witnesses said was menacing other straphangers before ex-Marine Daniel Penny placed him in a fatal chokehold aboard an F train in Manhattan.

Moncrieffe said the older man left the train at some point while he spoke to Lewis.

“I said I don’t know you, but I love and respect you,” Moncrieffe said he told Lewis.

“You’re a young black man. You look like me. I just want to make sure you get home safe.”

But Lewis offered a bone-chilling response, according to Moncrieffe.

“He then said to me, ‘Yeah you don’t know me, but I’m going to make sure you remember me,'” Moncrieffe said.

With that, Moncrieffe retreated to the further end of the car with his back to the door — then felt a tap on his shoulder when the train doors opened behind him at the Fulton Street station.

Moncrieffe turned around to find Lewis standing there.

“He says ‘Thank you,’… and as he says ‘Thank you’ he slashes me down my face,” Moncrieffe said.

Moncrieffe could no longer see out of his left eye as the blood poured from his face, never saw the weapon.

He began screaming at passengers to stay on the train, fearing that Lewis would attack someone else on the platform.

Eventually, he made his way up the platform to the front of the train, where passengers helped him up the stairs and police took him to Kings County Hospital.

Lewis was arrested eight days later after police released surveillance footage from the station.

Moncrieffe learned his alleged attacker had recently been released from jail without bail by Judge Dale Fong-Fredrick — four days after he had been arrested and charged with strangulation, assault, harassment, and other counts stemming from an arrest on April 10th, after he allegedly choked his girlfriend and threatened her with a knife.

Lewis had been previously been arrested for shoving his girlfriend into a window in January and was also cuffed for misdemeanor assault after attacking an employee at a homeless shelter in August 2022, but he took a plea deal and was convicted of disorderly conduct in November, according to a law enforcement source.

Lewis has been arrested a total of nine times since 2016 for crimes including fare evasion in February 2016, robbery in July 2021, and most recently assault and strangulation this year.

Moncrieffe, who needed about 100 stitches after the attack, said he broke down while testifying before a grand jury on May 26, where he saw the photos of his injuries for the first time as they were presented to jurors, who decided to indict Lewis on multiple counts of felony assault, including assault with a weapon and assault with intent to disfigure.

“That’s the first time I saw how deep the cut was and how big my face was, so I just broke down on the stand,” he said.

Brooklyn District Attorney’s requested to have Lewis held on $50,000 cash bail or $100,000 bond, but Judge Patrick Hayes Lewis instead set bail at just $7,500.

Moncrieffe considers the amount far too low.

“If someone does step forward and bail him out for just $7,500, I think that’s just ridiculous,” Moncrieffe said.

“His crimes have only gotten worse.”

Moncrieffe said the deep cut to his face, which severed some of his facial muscles, means he now can’t smile, laugh, or cry without pain.

He doesn’t see himself ever taking the New York City subway again.

“I won’t feel comfortable and will have extreme anxiety going on the train. It’s just not safe,” he said.

He continued: “It sucks and at the end of the day we deserve to get to and fro without someone attacking us just because.”

A GoFundMe has since been started for Moncrieffe’s medical costs with a goal of $100,000, which has since raised over $70,000.