[5][6][7] In November 2019, T-Mobile announced that Sievert would be promoted from chief operating officer (COO) to CEO in May 2020 when John Legere stepped down.

[5] Sievert took control a month earlier than planned, on April 1, 2020, the same day T-Mobile closed its merger with Sprint.

[7][21] During this time, Sievert oversaw the "Un-carrier" marketing campaign, which sought to rebrand T-Mobile's public image[22] with a focus on no overage charges,[11] no contracts, unlimited data, and other offerings.

[29] In 2023, Sievert's total compensation at T-Mobile was $37.5 million, up 29% from the previous year and representing a CEO-to-median worker pay ratio of 521-to-1.

[31] In June 2020, Sievert decided to pull all T-Mobile advertisements from Tucker Carlson Tonight because of rhetoric that criticized the Black Lives Matter movement.