T-Mobile CEO John Legere will exit the company in May 2020, leaving COO and President Mike Sievert as his successor. T-Mobile CEO John Legere isn’t waiting to complete the carrier’s Sprint merger to announce his departure. On Monday, Legere announced to his 6.5 million Twitter followers that he’ll step down on May 1, 2020 and that COO and President Mike Sievert will be his successor.