At a crucial moment in late-night television, a new showrunner is taking over “The Tonight Show Starring Jimmy Fallon,” a onetime ratings winner that lost its lead in total viewers soon after the inauguration of President Trump.
A 27-year veteran of NBC, Jim Bell, will become the so-called executive in charge at “The Tonight Show” immediately, the network said on Wednesday.
Mr. Bell is taking over the program at a time when “The Late Show With Stephen Colbert” is not only dominating the late-night ratings, but also cutting into Mr. Fallon’s lead among a group of viewers prized by advertisers: adults between the ages of 18 and 49.
Mr. Fallon took over “The Tonight Show” from Jay Leno in 2014. His cheerful, enthusiastic style made him the leader in total viewers until the spring of 2017, when Mr. Colbert leapfrogged him in the Nielsen ratings.
Mr. Bell, a longtime executive producer of “Today,” is not only a producer but a bona fide network executive. He has a big title: president, NBC Olympics production and programming.
By installing him at “The Tonight Show,” NBC is employing a strategy that had success at CBS.
Two years ago, CBS hired Chris Licht, the executive producer of “CBS This Morning,” to take over Mr. Colbert’s then-struggling show. Like Mr. Licht in the time before he took that job, Mr. Bell has no experience in late night.
In the two and a half years since Mr. Licht started working with Mr. Colbert, the viewership for “The Late Show” has skyrocketed, while the number of people watching Mr. Fallon’s program has dropped precipitously.
At “The Late Show,” Mr. Licht helped the host play to his strengths as a political satirist, insisting that he focus on the news, which was Mr. Colbert’s bread and butter when he hosted “The Colbert Report” on Comedy Central. After Donald J. Trump entered the White House, Mr. Colbert started to build ratings momentum as Mr. Fallon lost viewers.
NBC officials insisted on Wednesday that Mr. Bell would not try to make Mr. Fallon emphasize the news of the day. Instead, the “Saturday Night Live” alumnus is expected to continue to lean into his fun-and-games persona.
In the new television season, which started a month ago, Mr. Colbert’s show is averaging 3.6 million viewers, dwarfing the 2.4 million who tune in for “The Tonight Show.” A saving grace for Mr. Fallon is that he has held on to a larger percentage of adult viewers under 50, the rating point that matters most to advertisers.
Even through his ratings struggles, Mr. Fallon has remained the most-watched late-night host among young adults. But his lead in that key demographic is slipping. The latest measure has “The Tonight Show” scoring just a tenth of a ratings point more than its CBS rival. That’s the closest the margin has been since Mr. Colbert took over “The Late Show” from David Letterman in 2015.
Before the arrival of Mr. Bell, “The Tonight Show” was run by a triumvirate of producers: Mike DiCenzo, Katie Hockmeyer and Gerard Bradford. On Monday, Mr. DiCenzo announced on Twitter that he was stepping down after 10 years with Mr. Fallon. Ms. Hockmeyer, who is to go on maternity leave soon, will stay on as a top producer. Mr. Bradford is expected to have a different producing role at the show.
Lorne Michaels, the creator and longtime executive producer of “Saturday Night Live,” will keep his title of executive producer of “The Tonight Show.” Mr. Michaels is also the executive producer of the program that follows it on NBC, “Late Night With Seth Meyers.”