A little more than a year after installing a new showrunner at its signature late-night franchise, “The Tonight Show Starring Jimmy Fallon,” NBC is switching leaders at the program again.
The network announced on Monday that Jim Bell, a 28-year veteran of NBC who took over as executive in charge of Mr. Fallon’s show in October last year, is leaving the show and the company. His last show will be Thursday.
It is a significant executive change at the network: Mr. Bell was not only tasked with improving the ratings of the network’s 11:35 p.m. franchise, he was also once in charge of its lucrative Olympics coverage.
In a statement, Mr. Bell said the network had asked him to take over Mr. Fallon’s program last fall while he was planning NBC’s coverage of the 2020 Summer Games in Tokyo.
“As has been the case for my nearly 30 years at NBC, I said yes to their request, adding only that I would commit for a year and we would review the situation at that time,” Mr. Bell said.
“The past year with Jimmy and the terrific team at the show has been a blast, and I will always be grateful for this opportunity,” he continued. “But after serious contemplation, I realized I did not want to extend my time at the show.”
Gavin Purcell, a former producer at Mr. Fallon’s “The Tonight Show,” will return as showrunner for an interim period, the company said. His first show will be next Monday.
Mr. Fallon once had a commanding lead among adult viewers under age 50, the demographic prized by advertisers. Now “The Late Show With Stephen Colbert” on CBS has taken a slim lead in that segment.
Mr. Colbert also has a commanding total viewership lead: His show is averaging 3.5 million viewers, compared with roughly two million for “The Tonight Show” and ABC’s “Jimmy Kimmel Live!”
The big blow to “The Tonight Show” came in the latest ratings report, which put the show in third place among total viewers. According to Nielsen, Mr. Fallon is now trailing Mr. Kimmel, who had been the perennial third-place host, by 3,000 viewers.