The housing honcho who is supposed to be tackling NYCHA’s lead crisis failed to protect the city’s private buildings from the toxic substance in his previous gig, a Post investigation has found.

Before Mayor Bill de Blasio tapped Vito Mustaciuolo as the agency’s general manager in January 2018, he oversaw the enforcement division at the city’s Department of Housing Preservation and Development — which bungled nearly 6,500 lead cases in less than two years, an analysis of records shows.

“It was just a complete and utter nightmare,” said Beverly Sutherland, who along with her husband and young daughter was uprooted for months while the Mustaciuolo-era HPD dithered over lead remediation in her Harlem apartment.

“This isn’t a toilet overflowing. This is potentially extremely toxic for a child, not to mention could create medical issues for other people.”

The city’s lead-regulation code — Local Law 1 — lays out a strict timetable for HPD to force landlords to repair or remove traces of the hazardous substance within 123 days of its detection.

But Mustaciuolo’s enforcement division blew past that deadline on an astounding 29% of the 22,472 lead violations it found across 8,194 apartments, a Post analysis of nearly two years of HPD records found.

That amounts to 6,481 lead violations in 2,503 city homes left untreated past the 123-day limit.

Those lapses were recorded between the launch of HPD’s online database in April 2016 and Mustaciuolo’s departure for NYCHA in January 2018 — in the wake of revelations that the public housing authority had covered up years of failure to comply with federally required lead checks.

When Mustaciuolo arrival at NYCHA amid the scandal, the city portrayed him as a longtime crusader against lead who helped write Local Law 1 in the first place.

“Mustaciuolo played a critical role in drafting and implementing the 2004 Local Law 1, the City’s chief ordinance for lead testing and remediation,” City Hall claimed in a press release trumpeting Mustaciuolo arrival at scandal-scarred NYCHA, echoing his biography on the Housing Authority’s Web site.

But more than a half-dozen people who pushed for the law told The Post they did not recall Mustaciuolo playing any meaningful role in crafting the bill — and a review of nearly 1,500 pages of testimony from seven committee meetings held at the time found not a single mention of his name.

“That’s misleading as hell,” said Pete Sikora, the then-organizing-director of the New York Public Interest Research Group who helped pressure the City Council to pass the 2004 legislation in the face of opposition from Mayor Bloomberg’s City Hall.

“If Vito was there, I don’t remember him, and he certainly did not say anything,” Michael McKee, whose Tenants PAC endorsed both of de Blasio’s mayoral bids. “As for the claim that Vito wrote Local Law 1, that’s a joke. Matt Chachere wrote it.”

Chachere — an attorney who fought a string of federal cases that forced New York to toughen its lead laws — questioned why the de Blasio administration would trust Mustaciuolo to get NYCHA on the right track given his record overseeing the city’s private housing stock.

“The whole big thing about NYCHA is that Vito was going to make sure NYCHA inspects its apartments,” said Chachere. “That’s fascinating, because he never made private landlords inspect their apartments when he was at HPD.”

Tenants like Sutherland can attest to that.

“I found HPD extremely difficult to deal with,” said the media executive, 49, who had to flee her rent-controlled Amsterdam Avenue apartment when agency inspectors found lead paint there in late 2016.

HPD ordered her landlord to get the lead out, but then left Sutherland, her husband, and their then-3-year-old daughter virtually alone to face a “nightmare” for eight months.

The family spent five weeks in a hotel on their own dime, before landing in another unit in the building while their home was effectively torn down and rebuilt.

When city inspectors did come out, they’d commiserate — but stopped short of doing much to help, she said.

HPD did eventually patch up some of the contractors’ shoddy work, but only after the family had been out of their home for eight months, and racked up six-figures in damages.

They’re now in court with their landlord seeking compensation.

A rep for HPD questioned what the numbers say about Mustaciuolo’s lead performance — until The Post laid out its methodology.

“There has been a 90-percent decrease in childhood lead exposure in New York City due in part to HPD’s aggressive enforcement efforts,” said the rep.

A mayoral spokeswoman insisted in a separate statement that during de Blasio’s time on the City Council, he “worked with Vito together to pass Local Law 1.”

“The Mayor has full confidence in Vito Mustaciuolo,” said the City Hall rep.

Sutherland said that, based on her experience, NYCHA residents saddled with lead-ridden apartments would do well not to share that confidence.

“I was able to borrow money from my mom to stay at the hotel,” she said. “How would someone without my resources handle what we went through?”