The mayoralty of Michael Bloomberg began on January 1, 2002, when Michael Bloomberg was inaugurated as the 108th mayor of New York City, and ended on December 31, 2013.

Bloomberg was known as a political pragmatist, and for a managerial style that reflected his experience in the private sector. Bloomberg chose to apply a statistical approach to city management, appointing city commissioners based on their expertise and granting them wide autonomy in their decision-making. Breaking with 190 years of tradition, Bloomberg implemented a "bullpen" open plan office, reminiscent of a Wall Street trading floor, in which dozens of aides and managerial staff are seated together in a large chamber. The design was intended to promote accountability and accessibility. At the end of Bloomberg's three terms, The New York Times said, "New York is once again a thriving, appealing city where [...] the crime rate is down, the transportation system is more efficient, the environment is cleaner."

In 2001, the incumbent mayor of New York City, Rudy Giuliani, was ineligible for re-election, as the city limited the mayoralty to two consecutive terms. Several well-known New York City politicians aspired to succeed him. Bloomberg, a lifelong member of the Democratic Party, decided to run for mayor as a member of the Republican Party ticket.

Voting in the primary began on the morning of September 11, 2001. The primary was postponed later that day because of the terrorist attacks on the World Trade Center. In the rescheduled primary, Bloomberg defeated Herman Badillo, a former Congressman, to become the Republican nominee. Meanwhile, the Democratic primary did not produce a first-round winner. After a runoff, the Democratic nomination went to New York City Public Advocate Mark Green.

In the general election, Bloomberg received Giuliani's endorsement. He also had a huge spending advantage. Although New York City's public campaign finance law restricts the size of contributions which a candidate can accept, Bloomberg chose not to use public campaign funds and instead self-financed his campaign. He spent $73 million of his own money on his campaign, outspending Green five to one. One of the major themes of his campaign was that, with the city's economy suffering from the effects of the World Trade Center attacks, it needed a mayor with business experience.

In addition to serving as the Republican nominee, Bloomberg had the ballot line of the Independence Party. Under New York's fusion rules, a candidate can run on more than one party's line and combine all the votes received on all lines. Green, the Democrat, also had the ballot line of the Working Families Party. Bloomberg also created an independent line called Students First whose votes were combined with those on the Independence line. Overall, he won 50 percent to 48 percent.

Bloomberg's election marked the first time in New York City history that two different Republicans had been elected mayor consecutively. New York City has not been won by a Republican in a presidential election since Calvin Coolidge won in 1924. Bloomberg is considered a social liberal: He is pro-choice, favors legalizing same-sex marriage, and is an advocate for stricter gun control laws.

In 2002, Bloomberg delivered bids from New York City to host both the Democratic and Republican nominating conventions for the 2004 presidential campaign. The city won the bid to host the Republican National Convention in 2004. The convention drew thousands of protesters, many of them local residents angry over the Iraq War and other issues. The New York Police Department arrested approximately 1,800 protesters, but most of the cases were later dismissed.