T-Mobile completed its $31 billion acquisition of Sprint today and announced that CEO John Legere has resigned from the carrier's top job a month sooner than planned.

With today's close, T-Mobile said it has "successfully completed its long-planned Chief Executive Officer transition from John Legere to Mike Sievert ahead of schedule." T-Mobile had previously said Legere would leave at the end of April.

"I had originally planned to stay on through the end of my contract on April 30, 2020, but it makes much more sense to transition this responsibility to Mike today," Legere said. Legere "will continue as a member of the Board of Directors for the remainder of his current term, through the Annual Meeting of Shareholders scheduled in June 2020," the carrier said.

Legere became T-Mobile CEO in September 2012 and turned the then-struggling mobile operator around with an "Un-carrier" strategy that pitched T-Mobile as a customer-friendly alternative to the AT&T/Verizon duopoly. "During his tenure, Legere engineered a turnaround of the company as T-Mobile completely disrupted the wireless industry and became the fastest growing company in wireless, capturing 80 percent of the industry's postpaid phone growth from 2013 to today," the company said.

Sievert, now officially the CEO, was previously T-Mobile's president and chief operating officer. He'll lead a very different company than Legere did in 2012. While Legere played up the carrier's status as an underdog fighting a duopoly, T-Mobile is now one of three dominant players in the wireless telecom market. Sievert said that Legere "changed what it means to be a CEO. Everything that T-Mobile has accomplished is the result of his vision for what a different kind of wireless provider could be."

T-Mobile said its approach won't change going forward. "T-Mobile customers should expect everything the Un-carrier has ALWAYS stood for: treating customers right, shaking up the status quo and being a force for good," the merger-close announcement said.

Pre-merger, T-Mobile had about 86 million total customers and Sprint had about 54 million.

Merger was approved with conditions

T-Mobile and Sprint were able to close their merger because of a federal judge's ruling against a lawsuit filed by attorneys general from thirteen states and the District of Columbia. The attorneys general contended the merger would reduce competition in the wireless telecommunications market and harm consumers with higher prices.

The Department of Justice approved the deal with conditions, notably that T-Mobile and Sprint sell assets to Dish Network, which plans to build a 5G network and replace Sprint as the fourth major carrier. The deal lets Dish purchase spectrum licenses, wholesale access to the combined T-Mobile/Sprint network, and Sprint's prepaid business, including subsidiaries Boost Mobile and Virgin Mobile.

The Federal Communications Commission approval of the merger requires T-Mobile to deploy 5G to 97 percent of the US population within three years and to 99 percent of Americans within six years. That includes deploying 5G to 85 percent of rural Americans within three years and 90 percent within six years.