The Illinois gateway regained the title it last held in 2004, and the mayor made the announcement this week at the end of the World Routes 2014 convention that took place from September 20-23.
He explains from January to August 2014, more than 580,000 flights took-off and landed at O'Hare, according to new figures from the US Federal Aviation Administration (FAA).
In 2014, Chicago's airports have experienced substantial growth in passenger traffic, especially for international traffic, and through the first half of the year, international passenger volume is up 8% at O'Hare (5.2 million passengers) and 15% at Midway (289,300).
In the last 18 months, O'Hare and Midway airports have welcomed six new international airlines and also added dozens of new destinations.
Mayor Emanuel, explains: “O'Hare isn't just the busiest airport in the world, it's an asset for the City of Chicago. These new gains will help us attract new businesses and solidify our place as the best connected city in the US and around the world."
Chicago Department of Aviation (CDA) commissioner, Rosemarie S. Andolino, says through the contacts made at World Routes, Chicago's airports will continue to "grow service and choices for travellers".