“Whaddya want?” growls Jimmy Breslin, emerging from his bedroom in his bare feet into the hallway of his New York apartment and pulling on a dressing-gown over rumpled pyjamas. The hard-boiled newspaperman, author and poet of the city’s mean streets looks and sounds like a man who’s got out of bed on the wrong side.

I tell him politely that I have come to interview him. “Harrumph!” he responds, demanding black coffee for both of us from his housekeeper. His wife, Ronnie Eldridge, apologises for his “rudeness”.

At least I made it up to their apartment.

When I first arrived at the swish midtown Manhattan building and told the receptionist that I had an appointment with Breslin, the young woman who called him up was ordered to describe me. I hear her say, “She’s small, with red hair and blue eyes.” She laughed at his response. I asked her what he said. “I know no redheads, but send her up anyway.”

So here I am in Breslin’s home office, where he writes every day because, he explains, he has bills to pay. Now 79 years old and in rude health, he’s an avid swimmer who breakfasts on a bowl of oatmeal. He is no longer the legendary drinker of yore – he and the late Norman Mailer were boozing buddies, famous barflies in the days when Breslin was the city’s best-sourced – and best-sauced – newsman.

“If there is one part of life that I can recall, it is anything that happened in a saloon,” he boasts. He’s in curmudgeonly mode. He sits down at his desk, gestures me to the one comfortable armchair and runs his fingers through his white hair. He snarls truculently, in his native Queens accent, “OK, whaddya wanna know?”

I love Jimmy Breslin; he’s my kind of newsman. He’s one of a dying breed: a true reporter, a Pulitzer prizewinner (in 1986, “for distinguished commentary”). He was in the kitchen of the Ambassador Hotel in Los Angeles in 1968, and saw Sirhan Sirhan shoot Bobby Kennedy dead. Breslin ended up sitting on Sirhan’s legs after police subdued the gunman.

He’s never liked being called a journalist, preferring reporter because “it sounds more like someone who works for a living”. Work meant getting out from behind his desk, pounding the streets, sitting in bars with ordinary folk and listening to their stories.

He’s never had to chase celebrities, only ambulances. Nobody has written more powerfully on behalf of the city’s minorities – the young immigrants who come to New York in pursuit of the American dream, only for it to become a nightmare of poverty and prejudice. Breslin reckons he’s visited more slums than Mother Teresa or Princess Diana.

In another era he might have been the model for a raucous scribe in a smoke-rich, spittoon-ringing newsroom in a Damon Runyon short story. Indeed, he’s written the best book about Runyon, Damon Runyon: A Life (1991). I’m here, though, to talk about his latest, The Mafia Rat: A True Story. It’s a pitch-perfect account of a seventysomething guy, Burt Kaplan, from Brooklyn, who raised a daughter, ran a garment business, trafficked in drugs and set up hits for the mob until he turned state’s evidence against two crooked New York City cops. He became the mob’s biggest canary, singing to a grand jury.

Breslin has written every sort of story, from how baseball’s New York Mets lost 120 games one season to investigating why Son of Sam serial killer David Berkowitz murdered strangers. Indeed, he was the recipient of a chilling letter from Berkowitz. When asked why “this dangerous fruitcake” wrote to him, he said, “What are you, crazy? Who else would he write to?” He’s spent a lifetime reporting on a world of guzzling, gossiping and gaming, inhabited by the mugs and molls of Manhattan.

He has carried on tipping a metaphorical fedora to the grit and chutzpah of those days, while keeping a beady eye on our violent, venal times, with gripping narratives. A typical, pithy Breslin sentence reads, “He’s doing a short bit in Attica for poor usage of a gun”. Or, of a failed Mafia hit on Godfather John Gotti, who left one of his men, Frank DeCicco, in his car while he was inside a club: “He tarried. The bomb did not. Mr DeCicco became dust.”

Today, he may have started off in grumpy old man mode – he drops the act after a few shots of caffeine – but he’s terrific company. “You gotta find somebody who makes you laugh,” he says, draining his coffee and yelling for top-ups for us both. “It’s a city of eight million, for God’s sake. Can’t you find a single laugh?”

For Breslin, Kaplan is “a figure out of all ages of crime, out of Dostoevsky, of the Moors Murders, of Murder Inc.”. A few words spoken by Kaplan on his Brooklyn porch sent men to their deaths. From 1986 to 1993 he was the middle man between crime boss Anthony “Gaspipe” Casso and Louis Eppolito, 60, and Stephen Caracappa, 67, two New York City police detectives who passed information about investigations to the mob and committed at least eight murders-for-hire.

In 2004, a third of the way through a 27-year prison sentence for conspiracy to sell marijuana, Kaplan turned state’s evidence against the two murderous cops. “Rot, mob cop scum!” screamed New York’s red-tops when the pair were jailed for life on March 6 this year. Eppolito, the nephew of a well-known mobster, Jimmy “the Clam” Eppolito, was sentenced to life plus 100 years; Caracappa received life plus 80 years. Each was fined more than $4m (£2.4m).

This Sopranoesque world of turncoats, turf wars, jailed Dons, goodfellas and bent cops has long been Breslin’s natural habitat, although he’s also written fiction, including The Gang That Couldn’t Shoot Straight (1970), which became a film starring Robert De Niro. He’s also published a memoir, I Want to Thank My Brain for Remembering Me, and The Church That Forgot Christ, about the clerical sex abuse scandals in Ireland and the United States.

The stocky, craggy-featured Breslin first discovered Kaplan on the witness stand in 2004 in the federal courthouse in Brooklyn. He says he was feeling hesitant about the trial and the book he planned to write about it. At an early hearing, Breslin watched the defendants come into the courtroom: “Eppolito fat and sad-eyed, Caracappa, a thin, listless nobody”. Breslin asked himself, “Am I going to write 70,000 words about these two?” Then the trial started and he was pulled out of his gloom. “An unknown name on the prosecution list, an old drug peddler, a lifelong fence, steals the show and turns the proceedings into something that thrills. Right away I think, ‘I have found my book’.” It was, he remembers, a morning of “excruciating excitement”.

Kaplan testified in simple sentences: one following the other to start a rhythm that was compelling to the jury’s ear. “Burt Kaplan saw into the shadows and understood what they were, for he had lived in them for so many years,” says Breslin.

Although the saga of Eppolito and Caracappa has been the subject of two other books, it is what is happening to the New York mob, for which Kaplan is the perfect metaphor, that interests Breslin. “He is probably the last true believer in the code of the Mafia,” one of his lawyers told Breslin. After doing so much evil, Kaplan believed his final act to be “atrocious and unforgivable”. That toughness, the lack of sentiment, is what Breslin admires about the Mafia rat. It’s not that the Mafia were good guys, just that they were honest about who they were, he believes.

So why did Kaplan do it?

“He had no choice: if he hadn’t, his former associates would have rolled over on him,” replies Breslin, explaining that in 1998 Kaplan had been sentenced to 324 months in prison. At the end of Eppolito and Caracappa’s trial, in 2006, he had served nine years. “That equals 118 months out of 324.” Since Kaplan walked free from the court three years ago, nothing has been seen or heard of him. Perhaps he’s sleeping with the fishes? “Who cares?” shrugs Breslin.

Meanwhile, New York’s mob is going out like The Sopranos – not with a bang but with a whimper. To Breslin, its diminishment represents a kind of loss. Not for the rule of law, but something more intangible. When one crime boss had his bankroll snatched on Park Avenue, it was “past anything we even imagined. They were mugging Mafia bosses on the street,” sighs Breslin. Not that he is any apologist for the mob – he once suffered a brutal beating at the hands of gangster family who didn’t much care for an article he’d written. Rather, the attraction for him has been to tell the stories that made the legend what it was, he explains, sipping his coffee.

“I keep hearing people talking about the end of the Mafia, but I don’t know what that means. I do know that gambling, which once was a glorious fountain of cash for the outfit, is now a government-owned lottery machine that buzzes in every newsstand in the city.”

When I mention The Sopranos, he says, “Today, aside from showmen, the only ones rooting for the mob to survive are FBI agents assigned to the squads that chase gangsters across the streets from the city.”

He recalls how, late one night recently, he watched his friend De Niro in a mobster comedy on TV. “I feel sorry for him, because these Mafia parts, at which he is so superb and which he could do for the next 30 years, will soon no longer exist.”

Jimmy Breslin grew up in poverty among many cheap, unworthy punks. It was a troubled childhood, he says, but shrugs it off, adding that it was a long, long time ago. He’s from the same mean streets, though, as mob bosses, such as John Gotti and Vito Genovese.

He says that because he grew up in Queens, “which nobody in the history of New York newspapers ever wrote about or even saw, I was reputed to be streetwise and tough. Which was untrue. I didn’t fight. I chased stories, not beatings”.

When he was a boy, Breslin says there were the five New York Mafia families, and he’d heard of some of them because he lived on 101st Avenue in Queens. Further up the street was a place called the Bergin Hunt and Fish Club, with a big plate-glass front window that had fish in it. Inside were men wearing hats and smoking, playing cards – it was the home club of the Gotti family.

“They were safe, almost completely protected by the ignorance of our times. The head of the FBI, J Edgar Hoover, declared there was no such thing as the Mafia. The Mafia agreed.”

It was so secret nobody knew it existed back in the day. Breslin was in grammar school –John Adams High School, Ozone Park, Queens – when a flower-decked house was pointed out to him where a woman had recently died. It belonged to Vito Genovese, he learned.

Later, as a 20-year-old newsroom copyboy – he began on the Long Island Press in the 1940s before he was 16 and had to lie to get work in papers – he was invited to lunch with Frank Costello, “The Prime Minister of the Underworld”. Costello told him, “This Mafia is a dream so they could sell it to the public in movies. It doesn’t exist. You’re starting off. I don’t want you to look silly.”

He has not looked silly; indeed, he has had long, triumphant stints writing about sports, crime and politics on both the defunct New York Herald Tribune – “the writer’s paper” – The New York Post and The Daily News, as well as being a columnist for Newsday and the Los Angeles Times.

His journalism was fuelled by what someone once described as “Olympic bouts of drinking”. He stares at the dregs of coffee in his mug and confides that he gave up alcohol in the early 1980s following an epic bender with Senator Daniel Patrick Moynihan, resulting in a hangover of such destructive proportions that he was crippled for three days. “I haven’t touched it since,” he says, adding that he also made “a bunch of money, which I blew”.

Breslin had six children – two daughters, four sons – with his first wife, Rosemary Dattolico. Then, in 1982, a year after her death from cancer, he married Ronnie Eldridge, a New York City councilwoman, and now has three stepchildren. In 1994, Breslin was operated on for a brain aneurysm and briefly lost his memory. The ensuing relief at regaining it led him to reminisce about his rags-to-riches life in his memoir, I Want to Thank My Brain for Remembering Me.

Three years later, he wrote a beautiful tribute in Newsday to his daughter, Rosemary, a screenwriter who died at the age of 47 of a blood disease so rare it still does not have a name, he says quietly, his voice full of emotion. Then in April this year his only surviving daughter Kelly, 44, a PR and marketing executive, died four days after collapsing in a Manhattan restaurant.

We drink more coffee and talk about some of Breslin’s greatest professional memories, ranging from taking a helicopter flight with the Beatles to being hungover at Martin Luther King’s “I Have a Dream” speech. There was also a momentous night, in 1991, when observing a night of violence between blacks and Hasidic Jews in Brooklyn’s Crown Heights, he was beaten, stripped to his smalls and robbed.

What was it like to report on the assassination of President John F Kennedy in 1963? “In Dallas, I thought that every minute of every hour I ever had worked had prepared me for writing about this – this is a marvellous emotion for you to have when a guy with a wife and two kids gets shot dead, I remember thinking afterwards.

“There were 3000 reporters at the funeral in Washington, DC and I knew I couldn’t perform the simplest act of this very simple business of reporting if I had to do it in these crowded circumstances. I went into the White House lobby and from nowhere I thought of the cemetery. I told Art Buchwald, who wrote for my paper, ‘I’m going to go over to the cemetery and get the gravedigger’. He thought it was a great idea.”

The gravedigger, Clifton Pollard, told Breslin it was an honour to dig Kennedy’s grave in Arlington cemetery, in front of the tomb of the unknown soldier. Breslin arranged to meet Pollard there at the funeral. He never showed. “All day Wednesday while the world watched Kennedy being put into the ground, the man who dug the hole was on the other side of the hill, digging graves for dead servicemen for $3.01 an hour.”

“You didn’t even want to see the funeral?” Breslin asked Pollard. “I tried to go over,” he replied. “But a soldier told me it was too crowded and he couldn’t let me through. I’ll get over there in a little bit. Just sort of look around and see how it is, you know. Like I told you, it’s an honour.”

That story is used in American journalism schools, notes Breslin. “It’s called the Gravedigger Theory. Still, today, when you have a great story, you have smart editors telling reporters, ‘Go find the gravedigger’. It came out of my brain on a big day, when I needed something strong, and it made me look like a million dollars. I thank my brain because I always figured that this was the only game; just tell the truth about what you see.”

Finally, was he ever nervous of the mobsters? “You want to be afraid of something, be afraid of being broke,” he laughs. Then he stands up and gives me a kiss and a bear hug.

The Mafia Rat: A True Story, by Jimmy Breslin, is published by Mainstream, priced £7.99.