Jay Leno's Tonight Show Has Lowest Ratings Ever 13.61K Richard Lawson 09/03/10 10:08AM Filed to: Television Television Jay Leno Tonight Show Conan O'brien Defamer Ratings Though he's gained on him overall, Leno's lost 23% of Conan O'Brien's share of the coveted 18-49 y.o. demographic. All told, these are the worst ratings Tonight has had since it premiered, in the late 1850s. Still beating Letterman, though.