To migrate to the United States in 1957, he created a fake foundation, which supposedly sponsored him. He later claimed he forged the letters from prominent communist authorities guaranteeing his loyal return to Poland, as were then required for anyone leaving the country.

In 1962, Kosiński married an American steel heiress Mary Hayward Weir. They divorced four years later. Weir died in 1968 from brain cancer, leaving Kosiński out of her will. He fictionalized his marriage in his novel Blind Date, speaking of Weir under the pseudonym Mary-Jane Kirkland. Kosiński later, in 1968, married Katherina “Kiki” von Fraunhofer (1933–2007), a marketing consultant and a member of the Bavarian nobility.

Kosiński first worked at odd jobs to get by, including driving a truck, and he managed to graduate from Columbia University. He became an American citizen in 1965. He also received grants from the Guggenheim Fellowship in 1967 and the Ford Foundation in 1968. In 1970, he won the American Academy of Arts and Letters award for literature. The grants allowed him to write a political non-fiction book that opened new doors of opportunity. He became a lecturer at Yale, Princeton, Davenport, and Wesleyan universities.

Kosiński was friends with Roman Polanski, with whom he attended the National Film School in Łódź, and said he narrowly missed being at Polanski and Sharon Tate’s house on the night Tate was murdered by Charles Manson’s followers in 1969, due to lost luggage. His novel Blind Date portrayed the Manson murders.

American novelist David Foster Wallace described Steps as a “collection of unbelievably creepy little allegorical tableaux done in a terse elegant voice that’s like nothing else anywhere ever”. Wallace continued in praise: “Only Kafka’s fragments get anywhere close to where Kosiński goes in this book, which is better than everything else he ever did combined.” Samuel Coale, in a 1974 discussion of Kosiński’s fiction, wrote that “the narrator of Steps for instance, seems to be nothing more than a disembodied voice howling in some surrealistic wilderness.”

Kosiński appeared 12 times on The Tonight Show Starring Johnny Carson during 1971–1973, and The Dick Cavett Show in 1974, was a guest on the talk radio show of Long John Nebel, posed half-naked for a cover photograph by Annie Leibovitz for The New York Times Magazine in 1982, and presented the Oscar for screenwriting in 1982.

In June 1982, a Village Voice report by Geoffrey Stokes and Eliot Fremont-Smith accused Kosiński of plagiarism, claiming that much of his work was derivative of prewar books unfamiliar to English-speaking readers, and that Being There was a plagiarism of Kariera Nikodema Dyzmy — The Career of Nicodemus Dyzma — a 1932 Polish bestseller by Tadeusz Dołęga-Mostowicz. They also alleged Kosiński wrote The Painted Bird in Polish, and had it secretly translated into English. The report claimed that Kosiński’s books had been ghost-written by “assistant editors”, finding stylistic differences among Kosiński’s novels. Kosiński, according to them, had depended upon his freelance editors for “the sort of composition that we usually call writing.” American biographer James Sloan notes that New York poet, publisher and translator George Reavey claimed to have written The Painted Bird for Kosiński.

Journalist John Corry wrote a 6,000-word feature article in The New York Times in November 1982, responding and defending Kosiński, which appeared on the front page of the Arts and Leisure section. Among other things, Corry alleged that reports claiming that “Kosinski was a plagiarist in the pay of the C.I.A. were the product of a Polish Communist disinformation campaign.”

In 1984, Polanski denied Kosiński’s story in his autobiography. Journalist John Taylor of New York Magazine believes Polanski was mistaken. “Although it was a single sentence in a 461-page book, reviewers focused on it. But the accusation was untrue: Jerzy and Kiki had been invited to stay with Tate the night of the Manson murders, and they missed being killed as well only because they stopped in New York en route from Paris because their luggage had been misdirected.” The reason why Taylor believes this is that “a friend of Kosiński wrote a letter to the Times, which was published in the Book Review, describing the detailed plans he and Jerzy had made to meet that weekend at Polanski’s house on Cielo Drive.” The letter referenced was written by Clement Biddle Wood.

Kosiński responded that he never maintained that the book was autobiographical, even though years earlier he confided to Houghton Mifflin editor Santillana that his manuscript “draws upon a childhood spent, by the casual chances of war, in the remotest villages of Eastern Europe.” In 1988, he wrote The Hermit of 69th Street, in which he sought to demonstrate the absurdity of investigating prior work by inserting footnotes for practically every term in the book. “Ironically,” wrote theatre critic Lucy Komisar, “possibly his only true book … about a successful author who is shown to be a fraud.”

Soon after the book was published in the US, Kosiński was accused by the then-Communist Polish government of being anti-Polish, especially following the regime’s 1968 anti-Semitic campaign. The book was banned in Poland from its initial publication until the fall of the Communist government in 1989. When it was finally printed, thousands of Poles in Warsaw lined up for as long as eight hours to purchase copies of the work autographed by Kosiński. Polish literary critic and University of Warsaw professor Paweł Dudziak remarked that “in spite of the unclear role of its author,The Painted Bird is an achievement in English literature.” He stressed that, because the book is a work of fiction and does not document real-world events, accusations of anti-Polish sentiment may result only from taking it too literally.

He died by suicide on May 3, 1991 by ingesting a lethal amount of alcohol and drugs and wrapping a plastic bag around his head, suffocating to death. His suicide note read: “I am going to put myself to sleep now for a bit longer than usual. Call it Eternity.”

Kosiński’s novels have appeared on The New York Times Best Seller list, and have been translated into over 30 languages, with total sales estimated at 70 million in 1991.

According to Eliot Weinberger, an American writer, essayist, editor and translator, Kosiński was not the author of The Painted Bird. Weinberger alleged in his 2000 book Karmic Traces that Kosiński was not fluent in English at the time of its writing.