Can talk show host Larry King be replaced when he steps down?

When Larry King finally announced his retirement on Tuesday evening, it was fitting that one of the people who rang in to say goodbye was Nancy Reagan. "You didn't call to ask my permission!" joked the former first lady, who had made her share of appearances on the show. "Lots of love Larry – and I'll miss you!"

And so will millions of her countrymen – though, it must honestly be noted, somewhat fewer of them than if Larry had hung up his famous coloured braces a decade earlier. When Larry King Live went out on CNN for the first time in June 1985, Nancy's Ronnie was just starting his second term in the White House and the cable network itself was barely out of its infancy. Since then, the world, not to mention the American media industry, has been transformed. But not Larry King. He's a listed national monument – but also a throwback to a vanished era.

His place in the Guinness Book of Records is likely to remain as unchallenged as the marathon tennis match at Wimbledon the other day: a quarter of a century as host of the same show, in the same hour-long prime-time slot of 9pm, seven nights a week, every week, year round.

Like the seasons, presidents and celebrities came and went, wars and empires waxed and waned, but not Larry King. Over this career, he racked up over 40,000 interviews – not bad for a kid born into a modest Jewish family in Brooklyn as Lawrence Harvey Zieger the best part of 77 years ago.

From his early days he wanted to work in radio. He got his start in Miami, where he commentated on Miami Dolphins NFL games, before becoming a talk show host for the now defunct Mutual radio network. It was in Miami that he changed his name; Zeiger was "too ethnic" he was told. Then CNN came calling, and King simply transferred the radio call-in format to television.

His first guest was the then governor of New York state, Mario Cuomo, and during the next quarter century anyone who was making news for whatever reason sooner or later (usually sooner) would end up on the other side of the Larry King microphone. You never knew who you'd get. It could be Bill Clinton or Monica Lewinsky. It could be Frank Sinatra, growling and intimidating but oddly vulnerable as well. Or Mikhail Gorbachev – or a Marlon Brando run to seed, who presented King with the greatest conceivable test of an interviewer's aplomb by planting a big, fat kiss on his lips, during a show in 1994. Or you might see a disgraced business tycoon or a fallen sporting idol like Mike Tyson, or a main figure from the hottest tabloid true-crime drama of the moment.

Last week was as typical as any, with a guest list that included President Barack Obama, the basketball superstar LeBron James, Bill Gates of Microsoft, and the flamboyant pop diva and gay icon Lady Gaga, more than half a century King's junior.

Basically whoever King wanted as a "get" he got – with one very notable exception. "If we had God booked and O.J. [Simpson] was available, we'd move God," he told his viewers as the case of the former American football star accused of murdering his wife turned into an all-consuming US news event. But Larry never did manage to book O.J. The nearest he came was in 1995, the day after Simpson's controversial acquittal, as King was interviewing the lead defence attorney, Johnny Cochran. Suddenly, Simpson himself was on the line, correcting what he said were misrepresentations of what had happened, and answering a few questions from King.

The formula, though, was always the same. For the first half of the allotted hour, King chatted with his guest, before throwing proceedings open to callers. The way he did it, in his gravelly voice, became a trademark of the show: "Collinsville, Illinois, hello..." And for the first half of its life, the show could make big and very serious news indeed.

In early 1992, the Texas billionaire Ross Perot went to Larry King Live to announce his candidacy for the White House. It was anything but a cheap promotional stunt: Perot went on to win 19 per cent of the vote in the election that autumn – the most by a third-party candidate in 80 years. It could also be said that Larry King was responsible for the creation of NAFTA, the North American Free Trade Agreement between the US, Canada and Mexico. A few months after the election, Perot was back on the show for a joint interview-cum-debate with Al Gore, the then Vice-president, about the controversial trade deal which was strenuously opposed by Perot. Gore, representing the Clinton administration as it strove to push the bill through Congress, was deemed to have won the debate hands down. A few months later, NAFTA was duly voted into law.

Over the years, King changed physically, as we all do. Look back on the Sinatra interview of 1988, and he is eager and youthful, still the owner of a head of brown hair. Gradually however, the shirts became louder and the braces more colourful, while the man grew thinner and more grizzled, ever more intense as he hunched over the microphone.

With advancing age came gravitas as well – even though his personal life seemed to be a mixture of heart problems and broken marriages. He collected wives (eight at the last count) at the same rate as he did national media awards.

But in the Eighties and the Nineties above all, he was part of the news cycle. Almost invariably, a protagonist of the day's biggest story would turn up on the show. In those days, CNN had the cable market to itself, and there was no YouTube with instant replays of the best bits on your computer screen or cell-phone. Reporters were wise to watch for themselves. It helped of course that King, then as now, did not do opinion. He was an old-fashioned radio man at heart, whose forte was making a guest feel comfortable.

Never did he indulge in Paxman-esque grillings, and many complained that his questions were softballs. King was also known for not doing much homework beforehand on his interviewees, and sometimes it showed. But his seeming innocuity could prompt a guest to open up as he never would have to a more aggressive interviewer. Often, indeed, King seemed as thrilled as an ordinary viewer, at the fame or notoriety of the person he was talking to. That too was part of his charm.

Gradually however, the charm faded. For years the format has been showing its age, and for both King and CNN the competition has been growing ever fiercer. Fox News and MSNBC emerged as cable news rivals. The 24/7 news cycle meant that Larry King Live was merely one among scores of ever jostling outlets. On both left and right, a new generation of talk hosts, strident and opinionated, concerned not so much to elicit information as to project their own views, is making the running. The louder and more controversially they do so, the better the audience ratings.

Doggedly CNN tried to present itself as a lone voice of objectivity amid the cacophonous, partisan babble. But Americans, it would seem, prefer the partisanship. CNN's recent ratings slide has been alarming, and Larry King, for so long the pillar of its evening schedule, has not been spared. In the second quarter of this year, his show's audience tumbled to below 700,000 – its lowest level in a decade, and trailing far behind its rivals in the plum 9pm slot.

By the end, his departure had become inevitable. "I'm tired of the nightly grind," he reflected the other day, on the flight back to LA from Ohio, where he had just interviewed LeBron James. The network insists King is departing "on his own terms". In fact, whether he jumped or was given a polite push, is beside the point. His time was up.

Several replacements have been mooted, among them Piers Morgan, known here as a judge on America's Got Talent, and Katie Couric, the anchor on CBS Evening News. King's own preference would be the TV and radio host Ryan Seacrest – but what the heck? "I'm sure there are a ton of people that could do it," he said as he announced his going. "Come on, it's just Q & A." Maybe. But for better or worse, no one did Q & A quite like Larry King.