Here's a clip I can't embed but it's sure worth a trip over to YouTube to hear it. It's audio only of the first three minutes of Johnny Carson's first Tonight Show for October 1, 1962.
Quick explanation: At the time, the Tonight Show was an hour and forty-five minutes in some markets, 90 in others. This was because some stations ran a 15-minute late newscast before it and some had a half-hour. So the show would start at 11:15 (10:15 Central) and they'd do whatever they did, going to a commercial break at 11:28 or so. When they came back from that break, the show would have another opening billboard and it would more-or-less start over for those just joining it. Once Johnny was ensconced as host, he'd do his opening monologue in the first fifteen minutes and do some comedy bit or maybe even have on a first guest who was expendable for much of the country.
But Johnny felt his monologue wasn't expendable. Later on when he was successful and had some clout, he began to object to the arrangement. He stopped doing the first fifteen minutes of his own show, leaving that to Ed McMahon and bandleader Skitch Henderson. They'd host that part and then Johnny would appear at 11:30 and do his monologue for the full roster of stations. Eventually, the first fifteen went away completely.
On his first night, they wanted all of America to see his debut so Groucho Marx came in, did the first fifteen minutes and then introduced Johnny. And you'll hear that intro if you click here. I didn't know this still existed and I am, of course, curious if the rest of this episode does.