The Mafia big who was the first of his generation to spill the mob’s most precious secrets about its extensive reach into legitimate businesses, labor unions and illegal rackets in New York has died.

Alfonso “Little Al” D’Arco, who went from heist artist on the postwar Brooklyn streets to acting boss of the Luchese crime family before turning government witness in 1991, was 86. He died in witness protection earlier this month from complications due to kidney disease.

D’Arco’s decision to abandon his Mafia oath helped inspire other high-ranking turncoat mobsters, including Salvatore “Sammy Bull” Gravano, right-hand man to John Gotti, and a dozen others. All told, their collective testimony sent scores of mobsters to prison and helped dislodge New York’s Cosa Nostra from some of its richest prizes.

By the time D’Arco and other cooperators were done testifying, the powerful economic and criminal presence in the city known as the Five Families had been reduced to small groups of aging wiseguys assisted by young wannabes who displayed little talent for organized crime.

Most of those subsequent defectors, including Gravano, were facing the likelihood of life in prison when they decided to cooperate. D’Arco, however, was not under indictment for anything. The feds didn’t even know much about him. But he had a more immediate reason to want out: At the time, Little Al believed that the murderous chieftains at the top of the Luchese family hierarchy — a duo of Vittorio “Vic” Amuso and Anthony “Gaspipe” Casso — had marked him for death, a threat that extended to his wife and children as well.

D’Arco had firsthand knowledge of how suddenly and viciously Amuso, the official boss of the family, and Casso, his underboss, could turn on their longtime partners in crime.

As their handpicked acting boss, D’Arco had been ordered by Amuso and Casso, both of whom were fugitives from racketeering charges at the time, to arrange the murders of a dozen men aligned with the Luchese family. All were alleged by the bosses to be potential informants.

In a three-year spasm of violence between 1988 and 1991, Luchese gunmen killed a total of 17 men, their bodies left sprawling in underground garages, dumped into landfills, jammed into car trunks or secretly cremated. At the bosses’ direction, a dead canary was stuffed into the mouth of one Luchese veteran after he was assassinated in Brooklyn.

The bird was supposed to send the message that the victim had been an informant. But that was untrue. In fact, most of the death sentences ordered by Amuso and Casso, D’Arco came to realize, had little to do with Mafia disloyalty. Instead, most of the murders were an effort to eliminate potential rivals and seize their often lucrative crime operations.

His decision to switch sides was also spurred by outrage at what D’Arco, a firm mob traditionalist, viewed as violations of long-standing Mafia rule, including targeting family members of mobsters. Just a few weeks earlier, Luchese soldiers had been dispatched to threaten the lives of the elderly parents of a mobster the bosses feared was turning on them.

D’Arco soon became convinced that his number was up as well. While attending a business meeting of Luchese captains in a posh Midtown hotel, he spotted a mobster secretly stashing a pistol in the bathroom.

Believing he was being set up to be killed, D’Arco blurted goodbyes and bolted from the hotel room. The next morning, he sent his wife, two daughters and youngest son away, and fled with his older son, Joseph, who was also a crime-family member.

From a hiding place on Long Island, D’Arco reached out to the FBI. Agents soon scooped him up and began debriefing him.

Out poured a vast stream of mob knowledge. D’Arco detailed the loan-shark arrangements, the union locals and the mobsters who controlled them, the monthly payoffs from construction companies, the hierarchies of each crime family, even a list of more than two dozen factions in the Sicilian Mafia.

He stood just 5-foot-7, hence his nickname, and had an accent straight out of the Dead End Kids. He served two terms in prison, including a stretch in Sing Sing for stolen stock certificates in the early 1960s. That experience, along with a later federal conviction in the 1980s for heroin sales, served as a kind of graduate school for the aspiring gangster. Throughout, he had paid close attention to the business of the Mafia, and many of his stories shocked government officials.

He told how the mob had divided the proceeds from the scrap iron generated by the demolition of the old elevated West Side Highway; the mob had also profited from the construction of a huge nuclear power plant. Mob penetration of the construction industry remained so deep, he told them, that even a concrete contractor then working on federal projects and who had allegedly been vetted for mob ties was paying $20,000 each month to his crime family.

“He was the best,” Michael Campi, a former FBI supervisor who spent 20 years chasing wiseguys in New York and was one of those conducting the debriefing sessions, told us for the book “Mob Boss,” a biography of D’Arco. “Al D’Arco was the most significant made member to cooperate. He really built that bridge for others to cross.”

Born in north Brooklyn on July 28, 1932, Al D’Arco was raised a few blocks from the Navy Yard, then a hub of underworld hustles. An uncle was in the mob, but his father was a businessman, running a small basement dye shop for the garment industry. Above the shop, however, a local hoodlum had a bookmaking operation and the youngster was awestruck by the nattily dressed men who came and went.

“I’m still a mobster, but I’m an outlaw, that’s all. It’s not like they throw you out of the mob when you flip. You’re just considered an outlaw. That’s what I am.”

D’Arco committed his first crime at the age of 12, when he robbed a furniture factory across the street from his family’s home on Kent Avenue, a deed that earned a beating from his father. But he was soon working for a local candy store operator whose sideline was gambling and peddling stolen goods.

His one detour into legitimacy came during an Army enlistment when he was shipped in 1951 to an airfield near the Arctic Circle in Alaska, where he did guard duty on a frozen tarmac watching over long-range bombers. Two years later, he was back on the New York City streets, looking for moneymaking schemes.

Among those who tutored him was an old-school mobster named Vincenzo “Jimmy Alto” Altomari, who ran high-stakes crap games and whose headquarters was a cafe on Mott Street. “Be low key. Don’t stand out,” were among the lessons Altomari taught him, D’Arco later said.

He took the lesson to heart. Unlike flashy Mafia contemporaries such as Gotti, who sauntered through the Little Italy streets with an entourage, D’Arco lived modestly and conservatively. His residence was a government-subsidized apartment on Spring Street. There were no comares — girlfriends — on the side. Until his death, he remained loyal to his wife, Dolores Pellegrino, who had swept him off his budding young gangster feet when they’d met at a Brooklyn nightclub on Flatbush Avenue in 1953.

His only indulgence was a little restaurant on Cleveland Place called La Donna Rosa. Billed as Ristorante Siciliana, it featured recipes that D’Arco, an amateur cook, had learned from his grandparents, such as disco volante alla cognac con funghi — ravioli stuffed with wild rabbit mousse and wild mushrooms.

Patrons included Robert De Niro, who studied the moves of the mobsters who dined there, and John F. Kennedy Jr., who sat alone, quietly enjoying lunch during breaks from his duties as an assistant district attorney at the nearby courthouse. The late mayor and gourmand Ed Koch was once turned away at the door by D’Arco. A roomful of gangsters were holding a confab there.

“I told him it was a private party,” D’Arco told us later.

After he changed sides and testified for the government, prosecutors who worked with him were steadily impressed with both his uncanny memory and his insistence on holding to what he knew.

In a string of appearances as a witness for the government, D’Arco delivered devastatingly detailed testimony that helped win convictions against leaders of all five crime families, including Amuso and the elusive Genovese boss, Vincent “Chin” Gigante.

Altogether, prosecutors credited him with helping to convict more than 50 mobsters.

“He was one of the toughest, most difficult witnesses I ever faced,” said Gerald Shargel, the veteran defense attorney who represented Amuso and others.

Aside from recalling meetings and murders, his testimony from the stand also sometimes lapsed into a kind of mob poetry. Asked how he became involved with the Mafia in one trial, he explained that “it was always around my neighborhood. It’s like you’re in the forest. The neighborhood is the forest and all the trees in it, well, a lot of the trees, were organized-crime men. It was a way of life.”

His shift from gangster to witness was just another phase in that life, he said.

“I’m still a mobster,” he told prosecutors. “But I’m an outlaw, that’s all. It’s not like they throw you out of the mob when you flip. You’re just considered an outlaw. That’s what I am.”

Still, sitting in a chain motel in middle America far from the Little Italy streets for a series of interviews for the book, Little Al D’Arco often waxed nostalgic about the world he had left behind. “I could be back to crime now, if I wanted it. Crime is crime. You don’t forget how to make a living.”

But he had given his word to the feds, he said, adding, “If I am starving, I won’t go back to it. I gave my word.

“But as far as living day to day? I take that life any f–king day of the week.”

Tom Robbins and Jerry Capeci are the authors of “Mob Boss: The Life of Little Al D’Arco, the Man Who Brought Down the Mafia.” This obituary first ran on ganglandnews.com.