Part of the Paris postwar literary movement in the 1950s and a companion to Beat writers in Greenwich Village, Southern was also at the center of Swinging London in the 1960s and helped to change the style and substance of American films in the 1970s.

Southern frequented the Cinémathèque Française in Paris and saw jazz performances by leading bebop musicians including Charlie Parker, Dizzy Gillespie, Bud Powell, Thelonious Monk, and Miles Davis, evoked in his classic "You're Too Hip, Baby".

He became part of the New York artists and writers 'salon' of his old friend Plimpton—who had also moved back to New York— frequenting the Cedar Tavern, rubbing shoulders with writers James Jones, William Styron, Norman Mailer, Philip Roth, Harold "Doc" Humes, Jack Gelber, Jules Feiffer, Blair Fuller, Gore Vidal, Kenneth Tynan, the Aga Khan, the cast of the British comedy stage revue Beyond The Fringe, Jackie Kennedy, British actress Jean Marsh, and Tynan's first wife, Elaine Dundy, through whom Southern met satirist Lenny Bruce.

The editing process took much longer than expected: A drug bust led Trocchi to flee to the UK via Canada, leaving Southern and Seaver to finish the book, and editor Stephen Levine was recruited to assist.

This reportedly angered both Kubrick—who was notorious for his unwillingness to share writing credits[11]—and Peter George, who penned a complaint to Life magazine in response to a lavish photo essay on Southern published in the May 8, 1964 edition.

Jobs he considered included a proposed John Schlesinger screen adaptation of the Iris Murdoch novel A Severed Head, and a project called The Marriage Game, to be directed by Peter Yates and produced by the James Bond team of Harry Saltzman and Cubby Broccoli.

In the words of biographer Lee Hill, Southern spent the next six years in "an Olympian realm of glamour, money, constant motion and excitement", mixing and working with international literary, film, music, and TV stars.

Southern's work and his networking and socializing brought him into contact with many Hollywood stars, including Ben Gazzara, Jennifer Jones, Janice Rule, George Segal, Richard Benjamin, James Coburn, Peter Fonda, and Dennis Hopper and his wife Brooke Hayward.

The episodic "quasi-psychedelic burlesque" proved to be a chaotic production, stitched together from segments variously directed or co-directed by a team that included Joseph McGrath, Robert Parrish, Val Guest, John Huston, Richard Talmadge, and Ken Hughes.

Many writers contributed to the screenplay, including Southern (who wrote most of the dialogue for Sellers), Woody Allen, Wolf Mankowitz, Michael Sayers, Frank Buxton, Joseph Heller, Ben Hecht, Mickey Rose, and Billy Wilder.

Southern became close friends with photographer Michael Cooper, who was part of the Rolling Stones' inner circle and who shot the cover photos for the Beatles' Sgt Pepper's Lonely Hearts Club Band LP.

Southern's old friend Ted Kotcheff was then approached, but at this point, the project stalled – under the British censorship regulations of the time, the treatment had to be submitted to the Lord Chamberlain,[citation needed] who returned it, unread, with a note attached that said: "I know this book and there is no way you can make a movie of it.

Through the winter of 1966–67 he also began work on the screenplay for Roger Vadim's Barbarella, and he contributed to a TV version of The Desperate Hours directed by Ted Kotcheff and starring George Segal and Yvette Mimieux.

Although it brought Hopper and Fonda great financial and artistic rewards and helped to open up the Hollywood 'system' for young independent producers, little of the profit was shared with Southern, and the true extent of his contributions was repeatedly downplayed by the other principals.

The Magic Christian was one of Peter Sellers' favorite books—his gift of a copy to Stanley Kubrick led to Southern being hired for Dr. Strangelove—and a film version of the book had long been a dream project for the actor, who intended to play the lead role of Guy Grand.

At Sellers' request, a draft by Southern and director Joseph McGrath was re-written by Graham Chapman and John Cleese, two young British TV comedy writers who soon became famous as members of the Monty Python team.

Biographer Lee Hill suggests that Southern was a functioning alcoholic and that his image was largely based on his occasional public appearances in New York, partying and socializing; in private, he remained a tireless worker.

In the late 1960s, Southern's spendthrift ways and lack of financial acumen led him into trouble and he was audited by the IRS on several occasions beginning in 1972, resulting in heavy tax bills and penalties.

For the rest of his life, Southern was repeatedly forced to take on work in order to pay tax bills and penalties, and on many occasions he struggled to keep up the mortgage payments on the East Canaan farm.

Southern's other unrealized projects during this period included an adaptation of Nathanael West's A Cool Million, and a screenplay called Merlin, based on Arthurian legend, which was written with Mick Jagger in mind for the lead role.

Because of his acute money problems (exacerbated by the IRS affair), Southern took an adjunct lectureship in screenwriting at New York University, where he taught from the fall of 1972 to the spring of 1974; although popular among students, he was ultimately dismissed for holding his classes in a local bar.

A new short story, "Fixing Up Ert", was published in the September 1974 edition of Oui magazine, and around this time Norwegian director Ingmar Ejve hired Southern to write a screenplay based on the Carl-Henning Wijkmark novel The Hunters of Karin Hall.

In 1977 and 1978 Southern was embroiled in a lengthy and chaotic attempt to make a film version of Burroughs' novel Junkie, but the project collapsed due to the erratic behavior of its principal backer, Jules Stein.

In August 1978 Southern wrote a skit called "Haven Can Wait" that was performed by Jon Voight, Allen Ginsberg, Bobby Seale, and Rip Torn at a benefit for Abbie Hoffman.

Although he continued to reside in northern Connecticut "beyond the commuter belt", Southern maintained his social life in New York with diligence; longtime girlfriend Gail Gerber often drove him to Studio 54 (where he cultivated a convivial acquaintance with co-owner Steve Rubell), parties hosted by George Plimpton, and other engagements.

Under the pseudonym of Norwood Pratt, Southern co-wrote the 1980 sci fi-themed hardcore pornographic film Randy: The Electric Lady; director Philip Schuman had previously adapted "Red Dirt" into an award-winning short.

A year later, he was hired by Saturday Night Live head writer Michael O'Donoghue (who had solicited contributions from Southern as editor of National Lampoon a decade earlier) to write for the 1981–82 series of the NBC show in his efforts to revitalize the then-foundering sketch comedy program.

Despite his longstanding acquaintance with O'Donoghue and his penchant for the alcohol, cocaine and cannabis that flowed liberally backstage, Southern had trouble fitting in stylistically with the younger writers; many of his ideas and sketches were rejected by the staff and new producer Dick Ebersol for being too subtle, sexually gratuitous, or overly political.

Flash and Filigree was reissued by Arbor House with a new introduction by Burroughs, and Sandy Lieberson (now at Fox) hired him to work on a script called Intensive Heat, based on the life of jewel thief Albie Baker.

Southern and Nilsson collaborated on several screenplays, including Obits, a Citizen Kane-style story about a journalist investigating the subject of a newspaper obituary, but the script was scathingly reviewed by a studio reader and was never given approval.