Last year, NBC was hopeful that a change in producer would reverse the fortunes of “The Tonight Show Starring Jimmy Fallon.”

It did not take.

The network announced on Monday that Jim Bell, the 28-year NBC veteran who took charge of the program last October, is leaving the show and the company.

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, but he was also once the executive in charge of its lucrative Olympics coverage.

In a statement, Mr. Bell noted that the network had asked him to take over Mr. Fallon’s show while he was planning NBC’s coverage of the 2020 Summer Games in Tokyo.