A Mayor Bill de Blasio donor-turned-felon testified in extraordinary detail Thursday that he and his businessman pals wrote the book on city corruption — buying off the Mayor’s Office and the Police Department using brazen pay-to-play tactics.

“We’re going to become significant contributors, but we want access,” Jona Rechnitz, 34, testified telling de Blasio fundraiser Ross Offinger after Hizzoner clinched the Democratic nod for mayor in 2013.

De Blasio soon paid Rechnitz a visit in his office, the disgraced businessman told jurors in Manhattan federal court.

De Blasio — who last year called his relationship with Rechnitz “not a particularly close’’ one — handed the wheeler-dealer his private cellphone number and email address, the witness said.

The pair then began chatting “at least” once a week about “different issues in the city” — as Rechnitz funneled about $160,000 to de Blasio’s campaign and pet political projects, said the government witness.

The pair’s discussions included “if he wins, who he should be appointing for certain positions,” Rechnitz said of the mayor.

Rechnitz said he had high hopes for the kinds of favors he could potentially receive.

“My mind was limitless,” he said.

Business pal “Jeremy [Reichberg] had told me in the days of Giuliani, people made a fortune.

“I was focused on making money, getting my name out there, becoming a big player in town. So I figured maybe I’ll buy an office building, and I’ll get the city as a tenant. Maybe I’ll need to get special permits to make residential developments.”

Rechnitz appeared as the star witness in the bribery trial of former city corrections union chief Norman Seabrook. He is accused of bribing Seabrook to get him to invest $20 million in union pension money in a pal’s ailing hedge fund.

But testimony quickly veered toward de Blasio, as Rechnitz was questioned about his ties to the ­administration.

He described himself as a “yes man” who was happy to open his pocketbook as long as he was ­receiving favors from City Hall.

“I always gave money, as long as I was seeing him produce results,” he said of Offinger. “Whenever we would call him for access or for a ­favor, we were getting the response that we expected and the results we were expecting.”

Rechnitz and Borough Park businessman Reichberg initially targeted the NYPD in their pay-to-play scheme, doling out gifts and cash to cops in return for favors. Then they set their sights on City Hall, Rechnitz said. “We had the police going for us — and now it was time to get into politics,’’ he testified.

He first met with Offinger, who faced scrutiny during a now-closed federal probe into Hizzoner’s fundraising practices in 2013.

During that meeting, Rechnitz and his pals — including Reichberg and Fernando Mateo, president of the New York State Federation of Taxi Drivers — made it clear what they were after, he said.

“We’re one group and we expect a lot of access and influence in the office,” Rechnitz said the group told Offinger. “And when we call, we want answers. When we reach out for things, we want them to get done.”

“What was Mr. Offinger’s response?” prosecutor Martin Bell asked.

“’OK. How much do you think you guys can get together?’” Rechnitz said Offinger replied. “And I had committed at that point, I think I said about $50,000 to $100,000.’’

Rechnitz was a cooperating witness in earlier probes into de Blasio’s campaign fundraising. While de Blasio wasn’t cleared, authorities ultimately decided not to pursue an indictment , and said multiple City Hall “transactions appear contrary to the intent and spirit” of election law.

Rechnitz, who in March pleaded guilty to making contributions in exchange for advantageous treatment from government officials, said the favors started pouring in.

De Blasio “told me to call if there’s anything I need, always be in touch, and he really appreciated my support and friendship,” the developer said.

“I emailed him on his personal email. We would chat. I’d go to events of his. He invited me to events and put me in very good-seated areas.

“He took my calls. I mean, we were friends.’’

Rechnitz started calling Offinger every time he needed a favor — including one involving a friend’s massive water bill and violations Rechnitz faced for a tenant subletting a residence on Airbnb.

Rechnitz proved to be a generous donor, shelling out $50,000 for de Blasio’s now-shuttered nonprofit Campaign for One New York, and giving another $9,900 to his 2013 mayoral campaign.

In 2014, Rechnitz donated another $102,300 toward a failed effort led by de Blasio to help Democrats wrest control of the state Senate.

Rechnitz said some of the dough was from straw donors, which is illegal.

“A couple of people in my office, I had them write checks, because I wasn’t allowed to give more than $4,950. And I reimbursed them for those donations,’’ Rechnitz said.

Rechnitz said he promised Offinger to hit target donations — and the fundraiser would stop by his office to check on the fundraising.

“I had a lot of pressure from him to bring that amount in,” Rechnitz said of the pledged amount.

Mayoral spokesman Eric Phillips hotly dismissed the felon’s claims.

“These are nothing but reheated, repackaged accusations that have been extensively reviewed and passed on by authorities at multiple levels,’’ Phillips said.

“The administration has never and will never make government decisions based on campaign contributions.”

Offinger’s lawyer, Harlan Levy, did not return a request for comment. Reichberg did not return a request for comment, either.

During his hourlong testimony, Rechnitz said the corruption even extended beyond the Big Apple.

Westchester County Executive Rob Astorino gave him and Reich­berg positions as police chaplains in exchange for their ­financial contributions — even though neither of them is a rabbi or a priest, Rechnitz said.

“It meant that I got my parking placard,” said Rechnitz, whose firm JSR Capital donated $15,000 to ­Astorino’s campaign in June 2013.

He said Astorino once approached him with a picture of a Rolex watch and asked for help in procuring it.

“I told him I’m happy to give it to him; he doesn’t have to buy it,” Rechnitz testified.

“He told me that he couldn’t take it as a gift. He had to pay something because that wouldn’t be allowed. It was a $7,000 to $10,000 watch, if I remember correctly.”

In the end, Astorino agreed to pay $1,000 to $2,000 — and Rechnitz covered the rest, he said.

A rep for Astorino called Rechnitz’s testimony “total contrived nonsense, adding that the executive bought the watch himself, and he’s “never been accused of any wrongdoing.

“He has the credit card receipt to prove it, which he provided to the authorities prosecuting Mr. Rechnitz,” the spokesman said.

The NYPD declined comment.

Mateo’s lawyer, Glen Kopp, said, “Fernando has not been charged with anything.’’

Additional reporting by Shawn Cohen and Yoav Gonen