The tower tops out at a symbolic 1,776 feet in honor of the year America signed the Declaration of Independence.
Installation of the 408-foot, 758-ton spire, which will serve as a world-class broadcast antenna, actually began in December.
The last of 18 pieces were hoisted to the top of the tower just more than a week ago and put into place Friday morning, to loud applause and cries of joy from construction workers assembled below.
"It's a pretty awesome feeling," Tishman Construction project manager Juan Estevez said from a temporary platform on the roof of the tower where he and other workers watched the milestone. "It's a culmination of a tremendous amount of team work...rebuilding the New York City skyline once again."
He said the workers around him were "utterly overjoyed."
The new tower is at the northwest corner of the World Trade Center site, which is well on its way to reconstruction with the 72-story Four World Trade Center and other buildings.
With the beacon at its peak to ward off aircraft, the spire will provide public transmission services for television and radio broadcast channels that were destroyed on Sept. 11, 2001, along with the trade center towers.
The Port Authority of New York and New Jersey, which owns the site, said the LED-powered light would be activated in the next few months.
"It's going to have a light that you can see from tens of miles away," Port Authority Vice Chairman Scott Rechler said. "And that light will change colors and in the next few months we are going to be activating that light, and it will be a beacon of hope just like the Statue of Liberty."
Overlooking the Statue of Liberty and Ellis Island, the high-rise is scheduled to open for business in 2014.
The current tallest building in the Western Hemisphere is the Willis Tower in Chicago. The world's tallest building, topping 2,700 feet, is in Dubai.