The timing, so close to the election itself, helped the president over the line.
As Obama celebrated, he fully understood how significant the mayor's backing had been.
Over the years, mayors of New York have become major political figures in American politics.
One of those, who appeared on the front of Time magazine on June 7, 1948, can claim to have the most amazing tale of rags to riches known to the office.
When a scrawny 20-year-old William O'Dwyer arrived at Ellis Island in 1910 with just a few dollars in his pocket, no one could have predicted the success he'd achieve.
The eldest of 11 children, he was born in the Mayo village of Bohola, near Kiltimagh, in 1890.
With his Irish education complete, it was decided that he would travel to Spain to study for the priesthood.
But the youngster soon realised that the life of a priest wasn't for him. After two years he left and set sail for New York.
In 1916, he married Irish-American Catherine Lenihan and enlisted in the New York police department. But the ambitious young man knew he was destined for higher things. He studied at night and qualified from law school in 1923 before going into private practice two years later. In 1932, he became a judge before being elevated to the position of district attorney of Brooklyn in 1939.
After World War Two, he was elected as the 100th mayor of New York. During the war, New York had ground to a halt but the savvy O'Dwyer had a plan to get it moving again.
"The mayor brewed up a storm of building, which spread across the city. Schools, roads, hospitals and housing sprung up everywhere," reported The New York Times.
In October 1945, O'Dwyer's wife died after a long illness and it's thought he didn't want to run for re-election in 1949.
However, influential Bronx political boss Ed Flynn convinced him to run by getting President Truman to agree to appoint O'Dwyer as ambassador to Mexico in 1950.
He won his 1949 re-election bid easily, despite accusations of corruption.
His second marriage to Texan fashion model Elizabeth Sloan Simpson in December 1949 caused a stir nationwide. When they wed he was 59 -- she just 33. But the couple split in 1951.
Soon after, a Senate Crime Investigating Committee charged O'Dwyer with appointing friends of racketeers to high office and accepting underworld money.
He strenuously denied the charges and avoided jail.
Despite pressure on Truman to recall O'Dwyer from Mexico, the ambassador kept his post.
A highly successful diplomat, the remarkable Irishman eventually returned to New York in 1960. He died of a heart attack on November 24, 1964.
In the following morning's New York Times, it read: "William O'Dwyer was a warm, gregarious, fallible human being who made real and lasting contributions to New York City. . . we may never see his likes again."