Doyle's memorable cry of "Mickey Mantle sucks!" during the cold turkey sequence was the source of much trouble for the film makers and their legal department. Producer Robert L. Rosen had to track down Mickey Mantle to obtain his permission for the reference. After a long phone call, Rosen flew out to Mantle's home in Dallas with a print of the film, which was screened for him and his lawyer. When Gene Hackman uttered the line, Mantle surprised Rosen not only by roaring with laughter but also insisting that they watched the rest of the film because both he and his lawyer were enjoying it so much. Mantle later happily signed a release waiver and the line stayed in the film.
This was the first major motion picture sequel to simply have a number, or roman numeral (II), in the title, as opposed to "Part II," like the previous year's The Godfather: Part II (1974) (the first major motion picture to have a "Part II" in the title).
In planning the climactic chase in which Doyle pursues Charnier across Marseilles, director John Frankenheimer wasn't aware that Gene Hackman suffered from knee problems. Despite this, Hackman went ahead and filmed the entire chase without a double, badly inflaming his knee by the time he was through. He has said that Doyle's expressions of pain and determination as the chase progressed didn't require much acting.
The heroin processing lab was built by the Corsican mafia, and was so realistic that the entire set had to be guarded by French police when it wasn't in being used by the film crew. The mafia also advised on the methods used by drug smugglers to get heroin in the US (concealing the drug in freighter weights) and, according to John Frankenheimer, organized the permits for the traffic jam during the chase at the end of the film.
Eddie Egan, the real-life New York cop who was the basis for the character of Jimmy Doyle in The French Connection (1971), really did have a try-out for the Yankees in his youth, and played alongside a then-unknown Mickey Mantle.
According to producer Robert L. Rosen, Pete Hamill's uncredited rewrite of the screenplay took place over three days shortly before filming began, and virtually all dialog spoken in the movie was written by Hamill.
For the French version of the film, the character of Poppey was given a thick American accent in order to allow sequences where he encounters linguistic problems to make sense, not without occasional absurdities (for example, the main character has no trouble in arguing extensively and in an articulate way in French with local policemen but can't find a decent way to order a simple glass of whiskey).