Mayor Bill de Blasio called him a “liar and a felon.”

The man, Jona S. Rechnitz, was a wealthy real estate scion who made large donations to the mayor’s political campaigns and causes, gaining access to key officials, including Mr. de Blasio.

But Mr. Rechnitz became an instant pariah in City Hall after pleading guilty in 2016 to corruption-related charges. He admitted that those contributions — as well as direct bribes given to police officials — were a means to gain influence.

His admissions formed the core of a cooperation agreement in which he became a key government witness in three federal corruption trials, leading to the convictions and guilty pleas of half a dozen people, including a powerful correction officers’ union boss, a hedge fund mogul, a police official and a Brooklyn businessman.

Mr. Rechnitz became, according to federal prosecutors in Manhattan, “one of the single most important and prolific white-collar cooperating witnesses in the recent history of the Southern District of New York.”