Cohn was born to a working-class Jewish family in New York City. His mother, Bella Joseph, was from Russia, and his father, Joseph Cohen, was a tailor from Germany. After working for a time as a streetcar conductor, and then as a promoter for a sheet music printer, he got a job with Universal Pictures, where his brother, Jack Cohn, was already employed. In 1919, Cohn joined his brother and Joe Brandt to found CBC Film Sales Corporation. The initials officially stood for Cohn, Brandt, and Cohn, but Hollywood wags noted the company's low-budget, low-class efforts and nicknamed CBC "Corned Beef and Cabbage." Harry Cohn managed the company's film production in Hollywood, while his brother managed its finances from New York. The relationship between the two brothers was not always good, and Brandt, finding the partnership stressful, eventually sold his third of the company to Harry Cohn, who took over as president, by which time the firm had been renamed Columbia Pictures Corporation.
Most of Columbia's early work was action fare starring rock-jawed leading man Jack Holt. Columbia was unable to shake off its stigma as a Poverty Row studio until 1934, when director Frank Capra's Columbia comedy It Happened One Night swept the Academy Awards. Exhibitors who formerly wouldn't touch Columbia product became steady customers. As a horizontally integrated company that only controlled production and distribution, Columbia had previously been at the mercy of theater owners. Columbia expanded its scope to offer moviegoers a regular program of economically made features, short subjects, serials, travelogues, sports reels, and cartoons. Columbia would release a few "class" productions each year (Lost Horizon, Holiday, Mr. Smith Goes to Washington,The Jolson Story, Gilda, All the King's Men, etc.), but depended on its popular "budget" productions to keep the company solvent. During Cohn's tenure, the studio always turned a profit.
Cohn did not build a stable of movie stars like other studios. Instead, he generally signed actors who usually worked for more expensive studios (Wheeler & Woolsey, Cary Grant, Katharine Hepburn, Mae West, Humphrey Bogart, Dorothy Lamour, Mickey Rooney, Chester Morris, Warren William, Warner Baxter, Sabu, Gloria Jean, Margaret O'Brien, etc.) to attract a pre-sold audience. Columbia's own stars generally rose from the ranks of small-part actors and featured players (Jean Arthur, Rita Hayworth, Larry Parks, Julie Bishop, Lloyd Bridges, Bruce Bennett, Jock Mahoney, etc.). Some of Columbia's producers and directors also graduated from lesser positions as actors, writers, musicians, and assistant directors.
Cohn was known for his autocratic and intimidating management style. When he took over as Columbia's president, he remained production chief as well, thus concentrating enormous power in his hands. He respected talent above any personal attribute, but he made sure his employees knew who was boss. Writer Ben Hecht referred to him as "White Fang." An employee of Columbia called him "as absolute a monarch as Hollywood ever knew." It was said "he had listening devices on all sound stages and could tune in any conversation on the set, then boom in over a loudspeaker if he heard anything that displeased him." Throughout his tenure, his most popular moniker was "King Cohn."
Moe Howard of the Three Stooges recalled that Cohn was "a real Jekyll-and-Hyde-type guy... socially, he could be very charming." Cohn was known to scream and curse at actors and directors in his office all afternoon, and greet them cordially at a dinner party that evening. There is some suggestion that Cohn deliberately cultivated his reputation as a tyrant, either to motivate his employees or simply because it increased his control of the studio. Cohn is said to have kept a signed photograph of Benito Mussolini, whom he met in Italy in 1933, on his desk until the beginning of World War II. (Columbia produced the documentary Mussolini Speaks in 1933, narrated by Lowell Thomas.) Cohn also had a number of ties to organized crime. He had a long-standing friendship with Chicago mobster John Roselli, and New Jersey mob boss Abner Zwillman was the source of the loan that allowed Cohn to buy out his partner Brandt. Cohn's brash, loud, intimidating style has become Hollywood legend and was reportedly portrayed in various movies. The characters played by Broderick Crawford in All The King's Men (1949) and Born Yesterday (1950), both Columbia pictures, are allegedly based on Cohn, as is Jack Woltz, a movie mogul who appears in The Godfather (1972).
In his own way, Harry Cohn was sentimental about certain professional matters. He remembered the valuable contributions of Jack Holt during Columbia's struggling years, and kept him under contract until 1941. Cohn hired the Three Stooges in 1934 and, according to Stooge Larry Fine, "he thought we brought him luck." Cohn kept the Stooges on his payroll until the end of 1957. Cohn was fond of what he termed "those lousy little 'B' pictures," and kept making them, along with two-reel comedies and serials, after other studios had abandoned them.
According to biographer Michael Fleming, Cohn forced Curly Howard of the Stooges to keep working after suffering a series of minor strokes, which coincided with a further deterioration of Howard's health and his eventual retirement.
Cohn expected, or at least asked for, sex from female stars in exchange for employment (although similar stories were connected to many producers in Hollywood at the time). Harry Cohn's relationship with Rita Hayworth was fraught with aggravation. Hayworth's biography If This Was Happiness, describes how she refused to sleep with Cohn and how this angered him. However, because Hayworth was such a valuable property Cohn kept her under contract because she made him money. During the years they worked together, each did their best to irritate the other despite their lengthy work relationship which produced good results. Cohn wanted to groom Mary Castle as Hayworth's successor. Kim Novak, another Columbia star, reportedly endured similar treatment from Cohn. When Joan Crawford was subjected to Cohn's advances after signing a three-picture contract with Columbia, she quickly stopped him by saying, "Keep it in your pants, Harry. I'm having lunch with Joan and the boys [Cohn's wife and children] tomorrow."
In a BBC documentary, Sammy Davis, Jr. "The Kid in the middle", it was disclosed that Columbia Studios chief, Harry Cohn, in order to end Kim Novak's relationship with a black man, had mobsters threaten Sammy Davis, Jr. with blinding or having his legs broken if he did not marry a black woman within 48 hours.
Cohn was married to Rose Barker from 1923 to 1941, and to actress Joan Perry (1911–1996) from July 1941 until his death in 1958. Perry later married actor Laurence Harvey. His niece was Leonore "Lee" Cohn Annenberg, the wife of billionaire publishing magnate Walter Annenberg of Philadelphia. Her father was Maxwell Cohn, brother of Harry and Jack Cohn.
Along with Jack Warner, Cohn was the last Hollywood movie mogul of the studio system era, retaining power after the departures of such rivals as Darryl F. Zanuck and Louis B. Mayer. He suffered a sudden heart attack in February 1958 at the Arizona Biltmore Hotel in Phoenix, Arizona, shortly after having finished dinner, and died in an ambulance en route to St. Joseph's Hospital.
Cohn and his funeral were the subject of the famous quote from Red Skelton, who remarked of Cohn's well-attended funeral, "It proves what Harry always said: give the public what they want and they'll come out for it." He is interred in the Hollywood Forever Cemetery in Hollywood.