The motor psycho charged with starting the biker-mob attack that left a young Manhattan dad beaten to a pulp flipped off photographers in court Sunday — as it emerged that earlier this year, he caught a break from a judge.

Career criminal Reginald Chance, 37, extended both middle fingers as he was hauled into Manhattan Criminal Court on gang-assault and other charges tied to the Sept. 29 beatdown of Alexian Lien.

Chance’s lawyer said he wouldn’t challenge allegations his client was caught on video using his chrome helmet to smash a window of Lien’s Range Rover in front of the victim’s wife and 2-year-old daughter.

But defense lawyer Gregory Watts bizarrely insisted that Chance’s rage was justified, given that Lien had just run over another biker while trying to flee the gang of motorcyclists.

“It is entirely reasonable to try and identify the driver after a motor-vehicle accident,” Watts said.

The lawyer also said the assault case was trumped up, because Lien, who received stitches to his face, never had to be hospitalized.

Chance — who has 21 prior arrests for charges ranging from drugs to robbery to gun possession — was ordered held in lieu $100,000 bond or $75,000 cash.

Records reveal junk justice left him free to raise hell after an April drug deal near a Brooklyn school. Chance had allegedly sold cocaine through a middleman to an undercover cop in Bed-Stuy. A warrant was later executed on an apartment Chance was in and turned up cocaine, pot and a loaded handgun.

Chance was busted and hit with gun and drug raps and faced more than 25 years’ prison.

But Brooklyn prosecutors dropped the gun rap when Chance was indicted. A source said the charge was weakened by the fact that Chance — who allegedly admitted the pistol was his — had not been the target of the search for the gun.

Under a sweetheart plea bargain overseen by Brooklyn Criminal Court Judge Suzanne Mondo, all the remaining felony drug charges against Chance were also dropped.

He wound up with a one-year conditional discharge for misdemeanor pot possession and was given 20 days of community service.

Chance also received a six-month suspension of his driver’s license — which cops say he ignored when he joined in last week’s 300-biker Hollywood Stuntz ride that ended with the attack on Lien.

Chance was so brazen that he even breezed into court the day after the attack — before being linked to the beatdown — to tell Mondo he had been staying out of trouble, court records show.

Mondo, reached at her home Monday morning, declined comment.

But a court spokesman said, “It is quite possible that this was a negotiated plea with the District Attorney’s Office.”

The Brooklyn DA’s Office declined to comment.

At Chance’s arraignment, his lawyer insisted that his client had nothing to do with the actual attack on Lien.

“After the window was smashed, he got on his motorcycle and left,” Watts said. “There are still photographs that confirm he was not there during the assault.”

Manhattan Assistant DA Samantha Turino said that although Chance “did not land any blows, “he was an active participant and made it possible by gaining entry to the vehicle for the assault.

“His license is currently suspended and he should not have been driving anyway,” she added.

In addition to gang assault, he was charged with assault, criminal mischief, criminal possession of a weapon and unlawful imprisonment.

Cops are still hunting for more suspects, including a man with a stickered helmet whose photo they released Sunday and asked the public for help in identifying. Police also released a new photo of a biker they had already identified as a “person of interest’’ and believe helped beat Lien.

Meanwhile, a police source said one of two off-duty cops who took part in the ride didn’t see the assault. The other, an undercover detective, is being investigated for standing by during the attack — and not even telling higher-ups that he was there until Wednesday night, more than three days after the incident.

Mayor Bloomberg, asked on MSNBC’s “Morning Joe” show about the undercover officer’s actions, said he didn’t want to “pre-judge” anyone, but added, “I think we all, no matter what your job is, have an obligation to help one another. And if you see somebody getting beaten up, you know, let’s go jump in and stop the fight.”

He then crowed, “The good news is we have very few events like this – crime is way, way down. It’s so low that it defies imagination. … Having said that, one crime is one too many. … Fortunately, nobody was killed.”

Democratic mayoral contender Bill de Blasio added on Curtis Sliwa’s radio show, “We need to be very, very tough on these biker groups. … I think NYPD should get out to them and let them know exactly what they’re walking into if they try this kind of thing.”

Additional reporting by Josh Saul, Larry Celona, Lorena Mongelli and Yoav Gonen