For 66 years now, The Tonight Show has reigned supreme over the late-night television talk show world. It is actually the longest-running late-night talk show ever and the all-time longest airing weekly episodic entertainment program in American history. It is by far a fixture of American pop culture.

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The show has seen multiple tweaks and alterations over the course of the past six decades, including (but certainly not limited to) name changes, location changes, set changes, and of course changes in who hosts the show. It begs the question as to how many hosts have there been and why - excluding current host Jimmy Fallon - did they all leave? Let us explain.

13 The Host: Steve Allen

Before it was The Tonight Show, it was Tonight with Steve Allen. As the creator of the show, Today developer Sylvester "Pat" Weaver recruited Allen personally, based on his experience in local Los Angeles radio and his time as a regular on the variety game show, What's My Line?

Many of the common late-night TV tropes - i.e. the "man on the street" interviews - were originally kicked off by Allen himself. He offered plenty of new innovations in the genre and was the first to make the late-night genre popular, to begin with.

12 Why He Left

Steve Allen became so popular on late-night TV, in fact, that NBC ended up offering him his own primetime TV slot. He accepted, stepped away from late-night, and thus The Steve Allen Show was born.

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The main selling point for producers was they thought this could beat The Ed Sullivan Show in the primetime ratings war, but it was all for naught, as both were beaten by Maverick. The show was canceled in 1960, though it briefly returned in 1961 for a short fall season run.

11 The Host: Jack Paar

As Steve Allen made his exit, Jack Parr made his entrance. Prior to taking over Allen's seat, he dabbled in Hollywood with some notable appearances, like playing Marilyn Monroe's boyfriend in Love Nest. He also did his comic routine on The Ed Sullivan Show.

Perhaps out of their own ongoing obsession with beating The Ed Sullivan Show in the ratings, NBC scooped up Parr with a theater show hook and ladder and appointed him the new host of the Tonight Show.

10 Why He Left

Jack Parr was actually one of the more controversial hosts in the show's history. In 1960, he famously walked off of the set right before air time out of protest when he found out NBC censored a joke of his about a "water closet" (slang for toilet). He did not return for three weeks until NBC apologized.

His passionate fights with the network and the regular everyday routine eventually made Parr "bone tired" of the grind (per his words to Dick Cavett years later), hence why he left altogether after 1962, though he also regretted leaving, citing it as a big mistake on his part.

9 The Host: Johnny Carson

Often viewed as the greatest late-night talk show host not just in the history of The Tonight Show, but in late-night - period, Johnny Carson's unprecedented 30-year run as host began after NBC saw the success Carson had in hosting ABC's Who Do You Trust?

After an initial meeting between both parties, Carson actually declined the offer from NBC over fears about the responsibilities the job held. They were finally able to convince him in February 1962, after failing to convince other possible replacements like Groucho Marx, Bob Newhart, and Jackie Gleason. Once Carson settled into the role, he aced it.

8 Why He Left

Finally, 30 years later, Carson decided to hang it up. While his previous predecessors left after finding new opportunities or struggling to manage the workload, Carson simply decided it was time to peacefully retire and exit the entertainment world in favor of enjoying his riches at home.

At age 66, Johnny Carson came to the conclusion that he should ride off into the sunset with his six Emmy Awards in tow. He ended up being replaced by Jay Leno, but interestingly enough, he viewed The Late Show's David Letterman as his true "rightful successor," as per the New York Post.

7 The Host: Jay Leno

There was a huge controversy surrounding who would replace Johnny Carson, as many people - including Carson himself - thought then-Late Night host David Letterman would - and should - get the gig. Instead, after spending the last six years popping up on The Tonight Show as a guest host whenever Carson called in sick, Jay Leno was appointed by NBC as the official host.

Regardless of what his critics thought, Leno was a ratings draw in late-night TV by being the leading late-night show and highest-rated late-night show at the time. Carson didn't want Leno, but the viewers did.

6 Why He Left

After over 15 years on the air as the host of The Tonight Show, Jay Leno decided he wanted to conquer new challenges, namely in primetime. Thus, The Jay Leno Show was set to air in the fall of 2009 and Jay Leno's (at the time at least) final episode of The Tonight Show as its host aired on May 29th of that previous summer.

The question hurdling up in the air upon the announcement was who could possibly try to replace Jay Leno? Enter Coco.

5 The Host: Conan O'Brien

While he was hosting Late Night, Conan O'Brien signed a contract with NBC in 2004 ensuring that whenever Jay Leno stepped down as Tonight Show host, Conan O'Brien would take his place.

That clause in the contract was finally exercised in the summer of 2009. Before Coco knew it, The Tonight Show was his and his alone to do with as he pleased. But not for long. The honeymoon phase for Conan's new role came and went just as fast as his tenure as Tonight Show host.

4 Why He Left

As it turned out, both the Conan run Tonight Show and The Jay Leno Show failed to rank very highly in the ratings department. To rectify this, it was decided that Jay Leno would go back to late night with a half-hour cut in half, so Conan could premiere closer to midnight.

Conan did not like this plan or the fact that he was given zero notice before it was announced. NBC gave him two options: either take the new 12:05 am time slot, or leave NBC. Conan received a $33 million payout, packed his bags, and moved to TBS for a rebranded version of his show.

3 The Host: Jay Leno (Again)

With Conan out of the picture, Jay Leno was allowed to return to The Tonight Show's chair. Seemingly in support of Conan over his exit, Leno actually lost viewers in his return, averaging closer to 4 million viewers during his return year compared to the 5 million who tuned in during his last stint. These were coined at the time as the worst ratings in Tonight Show history.

Still, even with a ratings bump, The Tonight Show remained the most-watched late-night talk show on the air, and so neither NBC nor Leno had anything to worry about it.

2 Why He Left (Again)

Jay Leno never officially retired from late night, or the entertainment world altogether. He was pushed out of The Tonight Show more than he bowed out gracefully. When it came time for Leno's contract to end in 2014, rather than negotiate to renew it, NBC decided to pull the plug and replace Leno with Late Night host, Jimmy Fallon.

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Because business is business, Leno respectfully took the blow on the chin (or jaw, if you like those sort of jokes about Leno) and had his final episode (for real this time) on February 6th, 2014.

1 The Host: Jimmy Fallon

This brings us to today, as Jimmy Fallon has remained the host of The Tonight Show ever since. The former Saturday Night Live alum's stint has received mixed reviews, as those who love him continue to champion his work, while those who hate him have repeatedly tried to "cancel" him, to no avail.

As of now, per an NBC press release from 2015 via The New York Times, Fallon is expected to remain in the role at least until his contract ends in the fall of 2021. For the next year or so at least, love him or hate him, Fallon isn't leaving any time soon.

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