"People should have fun," said Joe Plumeri, chief executive officer of Willis Group Holdings.
"This is a town of neighbors, a town of ethnicities, a town of nicknames," said Plumeri, who said embarrassing monikers were a sign of affection in the New Jersey town where he grew up.
"They can call it the Big Willie as far as I'm concerned," he told AFP.
The official name -- the Willis Tower -- has not proven popular with residents of the Windy City since the name change was announced in March.
An online petition protesting the change has garnered more than 36,000 signatures and a Chicago Tribune columnist bemoaned "would New York let this happen to the Empire State and Chrysler buildings?"
Plumeri, who calls the naming rights "priceless," said he understands the sensitivity and vows to win the city's affection by being a good corporate citizen.
Willis won the naming rights after negotiating to lease several floors of the 110-story black tower which dominates the city's skyline.
The naming right has been up for grabs for years after retailer Sears, Roebuck and Co. moved out in 1993.
First opened in 1973, the Sears Tower held the record for the world's tallest building for 25 years until the Petronas Towers in Kuala Lumpur, Malaysia were built in 1998.
Designed with a series of stepbacks that allow a wide base to support the narrower upper tower, the tower's 4.56 million gross square feet (424,000 square meters) would cover 105 acres (42 hectares) if spread across one level.
The new name caps off a series of significant changes.
A series of glass bays that extend out from the building were added to the Skydeck -- a major tourist attraction -- offering visitors an unobstructed view of the city some 1,353 feet (412 meters) down.
Dubbed the "Ledge," the bays reach out 4.3 feet (1.3 meters) from the 103th floor and are retractable so as not to interfere with the window washing equipment.
The building management has also recently announced a 350 million dollar sustainability plan to increase the building's energy efficiency and build and eco-friendly luxury hotel.