Former Mayor Ed Koch, who presided over New York City from 1978 - 1989, died in hospital on Friday.
Koch, 88, was credited with lifting New York from crushing economic crises to a level of prosperity that was the envy of other US cities. Under his leadership, the city regained its financial footing and underwent a building renaissance.
But his three terms in office were also marked by racial tensions, corruption among many of his political allies, the rise in AIDS and HIV, homelessness and a high crime rate.
In 1989, he lost the Democratic nomination for what would have been a record fourth term.
Mayor Michael Bloomberg said the flags at all city buildings would fly at half-staff in his memory.
"In elected office and as a private citizen, he was our most tireless, fearless, and guileless civic crusader," Mr Bloomberg said.
"His spirit will live on not only here at City Hall, and not only on the bridge the bears his name, but all across the five boroughs."
Koch had a quip for every occasion and once said he wanted to be mayor for life. His autobiography, a bestseller, was turned into an off-Broadway musical.
He remained a formidable figure in New York politics until his death, endorsing candidates and offering political commentary on the NY1 TV station. In 2010 he formed New York Uprising, a political action committee designed to fight corruption in state politics.