Legere was a nationally competitive runner in college and post-college and still competes in events with fellow employees. In 2004, he completed the Boston Marathon as a member of the Dana–Farber Cancer Institute Marathon Challenge team. In May 2016 in support of competitive running, he purchased ad space on Nick Symmonds’ shoulder for T-Mobile.

Legere was appointed CEO of T-Mobile US in September 2012, and has since been noted for T-Mobile’s “Un-carrier” approach to contract-free wireless networks.

Since 2016, he has pitched his ideas and asked people to sign up for T-Mobile service with a cooking show on Facebook Live, called “Slow Cooker Sunday” which, as its name implies, runs every Sunday. In 2018, he wrote a cookbook featuring recipes he used in slow cooker Sunday broadcasts titled “#SlowCookerSunday – Leadership, Life and Slow Cooking with CEO and Chef, John Legere”. The book includes a foreword from Martha Stewart, a friend of Legere’s. The proceeds of the book go to the charity Feeding America.

In November 2019, Legere announced via his Twitter account he would be stepping down as the CEO for T-Mobile US on April 30, 2020, when his current contract expires. Mike Sievert, who was chief operating officer at the time, was announced as Legere’s replacement. The change was originally planned to become effective on May 1, 2020. Following the merger with Sprint being completed early, Legere officially resigned as CEO on April 1, 2020.