I seem to be binging old 90-minute Tonight Shows here. This one is from November 7, 1975 and the guests are Gene Kelly, Shecky Greene, Ronnie Graham and Stockard Channing. This is before Stockard had done Grease and she wasn't on the show to plug anything in particular. Johnny would have on guests just because he or someone thought they'd be amusing conversationalists.
I call your attention to Ronnie Graham, who was a comedy writer and cabaret performer…and a very funny man. For a while, Johnny would have him on often to just play his silly songs. I met Mr. Graham briefly on a few occasions and he was just delightful to be around. This link will take you directly to his performance which I think is quite wonderful. (In case you don't know, his second song is about Abe Beame, who was the mayor of New York City from 1974 to 1977. In '75 when this show aired, the city was in the midst of a huge financial crisis and Beame's popularity rating was somewhere below that of flea-and-tick season.)
Or you can watch the whole show from the start below…