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Five years ago, Norman Seabrook was one of the most politically powerful figures in New York City, the longtime leader of the 20,000-member union representing correction officers.

For two decades, he had the ear of mayors, governors and lawmakers from City Hall to Albany to Washington. He was an unapologetic champion for his members and exerted extraordinary control over the city’s Correction Department, shaping policy and even influencing key management appointments in the city’s jails.

But on Friday, Mr. Seabrook stood before a federal judge in Manhattan and was sentenced to 58 months in prison, the final chapter in a remarkable fall from grace that began when federal prosecutors started asking questions about his handling of the union’s funds.

Mr. Seabrook, 58, was convicted in August in a corruption trial. The evidence showed that he had steered $20 million from the union, the Correction Officers’ Benevolent Association, into a risky hedge fund, in exchange for a promised kickback worth more than $100,000.

Ultimately, prosecutors said, Mr. Seabrook was delivered $60,000 in bills crammed inside a designer bag purchased from his favorite luxury goods store, Salvatore Ferragamo.

“It’s time Norman Seabrook got paid,” Mr. Seabrook had told Jona S. Rechnitz, a real estate developer who had arranged for the payoff, according to Mr. Rechnitz’s testimony at the trial in Federal District Court. Mr. Rechnitz had pleaded guilty and became a cooperating witness for the government.

Before imposing the sentence, Judge Alvin K. Hellerstein said that he believed Mr. Seabrook “was blinded by his own sense of his own importance, by his desire to benefit himself after benefiting others for so long.”

The hedge fund, Platinum Partners, ultimately went bankrupt and the union lost $19 million of its investment. The hedge fund’s former chairman, Murray Huberfeld, who has pleaded guilty to conspiracy and is to be sentenced next week, has agreed to voluntarily return $7 million to the union, court filings showed.

Mr. Seabrook was also ordered to pay restitution of $19 million, although the judge acknowledged it was not likely his income would ever produce much savings.

The union said in a statement on Friday that it would continue to “fight until every penny is recovered and our members are made whole.”

Before he was sentenced, Mr. Seabrook made an impassioned and defiant statement to the court, saying he never intended for union members “to lose a dime” on the hedge fund. “I never betrayed my members, judge, never,” he said.

“My life’s journey may have been interrupted, but it’s not over until God says it’s over,” he added.

Later, speaking outside the courthouse, Mr. Seabrook said he would appeal his case and win. Asked about his lack of contrition, he said, “How can you be remorseful for something that you didn’t do?”

Mr. Seabrook’s lawyer, Paul Shechtman, said the sentence was harsher than he had hoped for, “but the most important thing is there’s a real viable appeal.” Mr. Seabrook’s first trial, in 2017, ended with a hung jury.

Mr. Seabrook’s trials drew widespread attention, partly because of testimony that touched on corruption in the New York Police Department and on Mayor Bill de Blasio’s fund-raising efforts. Prosecutors had asked Judge Hellerstein for a slightly longer sentence, of 63 months.

“This was a crime of betrayal,” a prosecutor, Martin S. Bell, argued on Friday. He said Mr. Seabrook came to believe he was “bigger than the people he represented,” adding: “He was owed. He was entitled.”