Mike Sievert, who'd been president and chief operating officer, took the helm, effective immediately and ahead of schedule.
The combined company will operate under the name T-Mobile. Shares of the Bellevue, Wash., company at last check were up 2.6% to $86.07.
Legere had been CEO since 2012. T-Mobile said in a statement that during his tenure, he "engineered a turnaround of the company, as T-Mobile completely disrupted the wireless industry and became the fastest growing company in wireless, capturing 80% of the industry’s postpaid phone growth from 2013 to today."
A federal judge approved the long-delayed merger in February, rejecting a claim by a group of states that the proposed transaction would violate antitrust laws and raise prices.
As part of an agreement with the Department of Justice, Reuters reported, the combined wireless company will sell off assets to Dish Network (DISH) - Get Report, making Dish the fourth-largest provider in the U.S. wireless space.
"I’m incredibly proud of everything we have accomplished at @TMobile over the past 7 years – and even in these uncertain times, I know the possibilities for what we can bring to American consumers is better than it’s ever been," Legere said on Twitter. "This next part is bitter-sweet. I’ve been fortunate enough to lead this company & the amazing people who are part of it for 7+ years. But, it's time to hand the reigns over to @MikeSievert so that he can lead the new company as CEO from day 1."
Sievert said in the company statement that "all of us at T-Mobile owe John an incredible thank you for everything he’s done to get this company to where we are today."
"He has changed what it means to be a CEO," Sievert said.
Legere, who was originally set to leave at the end of April, will remain on the board for the rest of his term, through the annual meeting in June, the company said.