A year after NBC upended its late-night world, order has quietly been restored: Jay Leno is back on “The Tonight Show” and topping the ratings virtually every week.

Conan O’Brien, who was displaced as host of the show, has moved to cable television and the other late-night hosts are maintaining their positions.

Some NBC executives have pointed to Mr. Leno’s slow but steady resumption of late-night leadership as vindication of the decision in January 2010 to end his brief run in prime time on “The Jay Leno Show” and reinstate him as host of “The Tonight Show.”

Mr. Leno, NBC executives say, is proving that the classic late-night show can be broadly appealing and still bring in more young viewers than any other entertainment show in the same time slot.