Mayor Bill de Blasio praised the acquisition, calling it “an investment in the future of Brooklyn” and saying the price was fair.

“Our administration keeps its promises,” Mr. de Blasio, a Democrat, said in a statement. “When we commit to build a new park or a new school in a growing community, we deliver.”

The promise to build the park was actually made during the administration of Mr. de Blasio’s predecessor, Michael R. Bloomberg, a Republican turned independent who estimated the project’s entire cost at $60 million to $90 million. With the deal announced this week, the city will have spent more than $350 million on land and another $25.8 million on development, with much more still needed to clean up decades of pollution on the property.

In the end, what stood between the people and their park was Norman Brodsky, owner of the elusive last tract and the fire-damaged warehouse on it. Mr. Brodsky rebuffed the city’s initial $100 million offer for the property and was reportedly seeking as much as $325 million. While real estate experts had placed the land’s minimum value at $120 million to $180 million, the city argued that the only reason it was even worth that much was the proposed rezoning, which depended on the creation of the promised park.