A new wireless giant has entered the scene.

T-Mobile and Sprint announced the closing of their $30 billion merger on Wednesday, the result of a long-in-the-works effort by both companies to speed the progress of wireless technology and put up a fight against AT&T and Verizon, the two companies that have long dominated the industry.

As part of finishing the deal, John Legere, the boisterous, magenta-clad chief executive who led T-Mobile for nearly a decade, handed over leadership reins to his longtime second-in-command, the more buttoned-up Mike Sievert.

The new business, called T-Mobile, will have about 100 million customers. To keep them and add to their ranks, the company plans to quickly develop the fifth-generation wireless technology that will bring broadband-style service through the air and is seen as a critical component of the nation’s infrastructure. T-Mobile has said that deploying 5G would have taken much longer and cost much more without the addition of Sprint.

Upgrading the networks also makes T-Mobile a formidable challenger to AT&T and Verizon, Mr. Sievert said in an interview. “It used to be that customers were forced to choose: Do you want a better network? Or a better value? Now you don’t have to choose,” he said.