His path to fame came soon after he took over as T-Mobile’s C.E.O. in 2012, and the company embarked on its “Un-carrier” strategy. Consider T-Mobile’s position at the time: The year before, the company’s plan to sell itself to AT&T for $39 billion failed, leaving what was then the U.S.’s fourth-biggest wireless provider with an uncertain future. All T-Mobile had to show for itself was a multibillion-dollar break-up fee paid by AT&T and a bundle of network airwaves known as spectrum.

But from that, Mr. Legere sought to change the way U.S. wireless providers did business:

• Lowering prices

• Ending some long-term contract requirements

• Offering unlimited data plans

“The company took off like a rocketship and has sustained that momentum ever since the AT&T merger was blocked,” the analyst Craig Moffett of the research firm MoffettNathanson told me in an interview.

More on the legacy of Mr. Legere, from my and Cecilia’s story on the deal:

Those policies helped T-Mobile add nearly 40 million customers over the last five years, with 5 million new customers added last year alone. AT&T, Verizon and Sprint all followed suit, and in recent years the overall price of basic wireless plans has stayed flat or fallen, according to Obama-era regulators.

The Un-carrier campaign helped propel T-Mobile ahead of Sprint in 2015. As of Dec. 31., the company had 58.7 million retail subscribers, compared to Sprint’s 40.9 million. T-Mobile’s market value also outstripped that of Sprint’s, contributing to the structure of Sunday’s deal: Mr. Legere would become C.E.O. of the combined company, which would keep the T-Mobile name.

In an ironic twist, completion of the Sprint deal would put T-Mobile ahead of its former suitor, AT&T, in terms of retail subscribers. AT&T had 93.2 million at the end of 2017.

But Mr. Moffett said that the very success of Mr. Legere and T-Mobile could pose problems for the Sprint deal, since it proves that regulators’ opposition to the two companies’ effort to merge in 2014 was justified.