Shares of T-Mobile were up 0.2% at market close after opening down 1.5%.
Sievert has been the heir apparent as Legere was expected to step down as CEO once T-Mobile's merger with Sprint is completed. Legere's contract ends on April 30, 2020, according to the announcement. Leger tweeted Monday that the decision "has been under development for a long time."
CNBC's David Faber reported in July that Sievert was expected to take over as CEO once the deal closes. The $26 billion merger has cleared key hurdles on the federal level but still faces a legal challenge from a team of state attorneys general seeking to block the deal. The case will go to trial next month.
When T-Mobile and Sprint first announced their intention to merge in April 2018, the companies said Legere would lead the combined business as CEO, with Sievert continuing to serve as President and COO.
Legere's next move is still not known. CNBC and The Wall Street Journal reported last week that WeWork had spoken to him about taking over as CEO following Adam Neumann's ouster. But CNBC later reported that Legere is not taking the job, according to people familiar with the matter. On a conference call Monday, Legere denied that he was ever in talks to be the WeWork CEO.
Legere was named T-Mobile's CEO in 2012 and made a number of aggressive moves to increase the company's wireless subscribers as it faced steep competition from its larger rivals, AT&T and Verizon. As other carriers pushed customers to sign up for wireless plans with data caps, Legere created unlimited wireless plans for T-Mobile and offered to pay termination fees for customers who switched over. Eventually, the rest of the industry followed suit and offered unlimited plans as well.
Legere was also responsible for bringing the iPhone to T-Mobile for the first time. And he went through multiple merger talks with Sprint over the years before finally locking the deal down in April 2018. The company expects to complete the merger next year.