In the opening moments of the Warner Bros. movie “Gangster Squad,” the audience is introduced to the noirishly lit, bulging muscles of Mickey Cohen (finely sculpted by their real owner, Sean Penn), as Cohen is described in voiceover as a boxer from Brooklyn and “a Jew.”

It is the most humane glimpse of Cohen the movie offers. Penn took poetic license with the gangster’s myth and chose to personify him as evil incarnate and not the strange, charismatic, enterprising, image-obsessed, lawless germophobe he actually was. So, while critics have had some fun comparing Penn’s Cohen to a Batman or Bond villain, in real life, Cohen was less fearsome, more pitiable. Nevertheless, he remains an integral part of a Los Angeles’ mob legacy, which includes more than its fair share of racketeering Jews. Having had a hand in almost every major modernizing industry, one could say the Jewish mob made Los Angeles.

Cohen would be thrilled to see a Hollywood movie that hinges on his legacy — that is, this puffed-up, glitzy one that bares a scant resemblance to reality. Harvard-educated former Los Angeles Times reporter Paul Lieberman wrote the seven-part investigative series for the newspaper that formed the basis for his nonfiction book “Gangster Squad,” upon which the film is based. According to Lieberman, Cohen wasn’t as mythic as he’d have you believe. Although the book “Gangster Squad” has the dark debauchery and dangerous glamour of a James Ellroy novel, it is based on facts drawn from 16 years of painstaking research, which Lieberman undertook after receiving a call from Sgt. John O’Mara, the squad’s leader, in 1992.

“What interested me as I got into it is that, in a way, the noir era still defines Los Angeles,” Lieberman said during a phone interview last week. He was especially enthralled by Mickey Cohen’s reign — his rise to L.A. kingpin and his fall to cartoonish joke.

“In 1949, there was a new scandal almost every week,” Lieberman said. “Or a shooting. Mickey was a giant in the headlines. It may sound absurd, but you’d hear stuff like, ‘This is an alien invasion’ — and that was the police chief talking that language – or ‘Los Angeles is a maiden in distress, and you’ve got to save her!’ ”

The dramatic headlines were fitting for a man obsessed with his own image and obsessed with Hollywood. Cohen would often boast he knew “half the movie business” on a first-name basis — the likes of Marilyn Monroe, Clark Gable, Jean Harlow and Cary Grant. His notoriety helped cement his status as the king of vice, but his legend is merely one strain in Los Angeles’ sordid mobster past.

In his 2006 book “Supermob,” Gus Russo writes that “two types of power dominated the twentieth century: the visible, embodied in politicians, corporate moguls, crime bosses, and law enforcement; and the invisible, concentrated in the hands of a few power brokers generally of Eastern European and Jewish immigrant heritage.”

Los Angeles had both. There were the “hoods” (tough guys) and the “Supermob” (above-the-line lawyers, bankers and real estate investors), which the investigative reporter Brian Ross once delicately referred to as the “bridge between polite society and criminal society.” What they all had in common, according to Russo, was a “shared sense of entitlement regarding tax-free income.”

He dubbed them the “Kosher Nostra.”

The Jewish Supermob included names like Korshak, Arvey, Greenberg, Pritzker, Annenberg and Ziffren — some of which still command attention today. Although real estate became the ultimate (and legitimate) means through which they cemented their wealth and power, the Supermob’s connection to the movie business was another outlet for its influence. In fact, the lure of Hollywood money is what caught the attention of the granddaddy Chicago mobsters who were looking for ways to boost their own bottom line.

“For numerous reasons, Southern California was the ideal place for transplantation of the mob-Supermob alliance,” Russo writes of the historic power shift from Chicago to Los Angeles. L.A., he continues, “was known as a city receptive to both hoodlums and Jews.”

Russo also points out that when L.A. was incorporated as a city in April 1850, there were only eight Jews in a population of nearly 9,000 (although two of those Jews served on the city council). After the second world war, Russo explains, “Jewish newcomers were so predominant that by 1950 only 8 percent of adult Jews in L.A. had been born here.”

The best and worst of them came together at the Hillcrest Country Club, founded in 1920 on Pico Boulevard and Avenue of the Stars (right across from today’s Fox lot). Back then, Hillcrest was a playpen for the early Hollywood moguls — including Louis Mayer, Harry Cohn and the Warner brothers, to name a few — a kind of secular synagogue where the movie moguls could rub shoulders with the Supermob. Many became “friends”; everybody got rich.

By the mid 1940s, perhaps no individual more aptly embodied both hoodlum and Jew than Meyer “Mickey” Cohen, the inveterate antagonist in “Gangster Squad” (although Cohen was not a member of Hillcrest, his mentor, Bugsy Siegel was, though he lost that privilege after being indicted and incarcerated in the fall of 1940). Penn’s portrayal of Cohen as a ruthless, heartless brute is, at least, indicative of his power. At one point, Cohen was so influential that state and city law enforcement buckled under his authority. “Either they would go along with the program,” Cohen once wrote of California’s midcentury police commissioners, “or they would be pushed out of sight.” Russo also recounts that when Richard Nixon first decided to run for Congress, he demanded Cohen raise $75,000 for his campaign, “to assure Nixon’s leniency toward the local bookies.”

Cohen came a long way from the Brooklyn-born teenage boxer who wore a Star of David on his trunks. Life hardened him, and he morphed into a sober, somewhat paranoid germ freak who washed his hands at least 100 times a day. Tere Tereba, author of “Mickey Cohen: The Life and Crimes of L.A.’s Notorious Mobster,” observed, “No drugs, alcohol or tobacco touched his lips. He was addicted only to making money and spending it, and to the siren song of his own celebrity.”

By the time the Gangster Squad was through with Cohen, “Mickey becomes this self parody, promoting his own image, selling shares of his life story — almost like out of ‘The Producers,’ ” Lieberman said.

But in the movie, Cohen is fabulously wealthy, living in an enviable Brentwood manse. In life, however, he lived in a relatively modest home, which was eventually sold at auction for $40,000 after the IRS launched a tax-evasion case against him. His financial desperation became his downfall: Cohen even went so far as to form a relationship with the Rev. Billy Graham, stringing the Evangelist preacher along with promises that he’d convert, so long as someone could foot the bill for his troubles. And behind the scenes, of course, “He’s laughing at these people,” Lieberman said.

“By then he was this image-obsessed hoodlum who wants to play up being a hoodlum.”

But even as the prodigal parvenu, Cohen had his values. He loved animals — “he had dogs, not children,” Lieberman explained — and he once took a 12-year-old actress named Janet Schneider under his wing, inviting the aspiring Cincinnati native to Hollywood, where he introduced her to his friends in show business.

The girl’s father was footing the bill, of course, but Cohen “was very protective of her,” Lieberman said, “like a loving uncle.” He even sent her an autographed photo of the two of them from her visit, writing: “To my little girl Janet and my little friend, I just know that you can’t miss reaching the absolute heights — Love, Mickey.”

In the end, Cohen the hoodlum had unforgivably killed at least one person, served two prison terms for tax improprieties, was the target of more than 10 assassination attempts and been beaten in prison with a lead pipe — but, what the movie “Gangster Squad” leaves out is that he was also a human being.