It was the first R-rated movie to win the Academy Award for Best Picture since the introduction of the MPAA film rating system. It also won Academy Awards for Best Actor in a Leading Role (Gene Hackman), Best Director, Best Film Editing, and Best Writing, Screenplay Based on Material from Another Medium (Ernest Tidyman). It was nominated for Best Actor in a Supporting Role (Roy Scheider), Best Cinematography, and Best Sound. Tidyman also received a Golden Globe Award, a Writers Guild of America Award, and an Edgar Award for his screenplay.
In 2005, the film was selected for preservation in the United States National Film Registry by the Library of Congress as being "culturally, historically, or aesthetically significant".
The eventually successful casting of Rey as the main French heroin smuggler, Alain Charnier (irreverently referred to throughout the film as "Frog One"), resulted from mistaken identity. Friedkin had asked his casting director to get a Spanish actor he had seen in Luis Buñuel's French film, Belle de Jour, who was actually Francisco Rabal, but Friedkin did not know his name, and Rey, who had played in several other films directed by Buñuel, was instead contacted. After Rabal was finally reached, they discovered he spoke neither French nor English, and Rey was kept in the film.
The film revolves around the smuggling of narcotics between Marseilles, France and New York City. The film opens in Marseilles with a policeman staking out Alain Charnier, a French criminal who ostensibly works as a former stevedore-turned-shipping executive but is in fact involved in smuggling heroin from France to the United States. The French policeman is eventually killed by Charnier's henchman, Pierre Nicoli.
In New York City, detectives James "Popeye" Doyle and Buddy "Cloudy" Russo are also performing an undercover stakeout, with Doyle dressed as Santa Claus and Russo pretending to be a hot dog stand vendor. Eventually the suspect they are waiting for makes a break for it, and the detectives pursue him on foot. After catching up with their suspect and delivering a severe beating after the suspect cuts Russo on the arm with a knife, the detectives aggressively interrogate the man and eventually force him to reveal where his "connection" is based.
After Russo's arm injury is treated, Doyle convinces him to go out for a drink. At the nightclub they go to, Doyle becomes interested in two people: Salvatore "Sal" Boca and his beautiful young wife, Angie, who are lavishly entertaining several known Mob members involved in narcotics. Doyle persuades his partner to come along as they tail the couple; several scenes are shown establishing the fact that although the Bocas run a modest newsstand diner, their extravagant lifestyle includes nearly nightly trips to several nightclubs, as well as driving several different new cars, which indicates they may be involved in some sort of criminal activity. Eventually there is a link established between the Bocas and well-to-do lawyer Joel Weinstock, who is rumored to have extensive connections in the narcotics underworld (in a voice over exchange Popeye and Cloudy allude to a drug shipment from Mexico bankrolled by Weinstock).
Doyle and Russo then roust an African American bar in Bedford Stuyvesant where the majority of the patrons are in possession of low quality marijuana and other minor drugs. The rousting is a stunt for Doyle to find an informant (who he physically assaults to keep his cover) whom he then questions about an apparent shortage of hard drugs on the street; Doyle is told that there is word a major shipment of heroin is on its way. The detectives convince their supervisor, Walt Simonson, to pursue wiretapping the Bocas' phones and use several ruses to try to obtain more information on their subjects.
The film now centers on three main points: the criminals' efforts to smuggle drugs into the U.S. which is made easier when Charnier dupes his friend Henri Devereaux into importing an automobile into the U.S. (unbeknownst to Devereaux, the drugs are carefully concealed within the vehicle) and the eventual sale of the drugs to Weinstock and Sal Boca; the efforts of Doyle and Russo to shadow Boca and Charnier; and the conflicts the two detectives have with both Simonson (their superior) and a federal agent named Mulderig. Both Doyle and Mulderig openly dislike each other; Russo and Doyle feel that they can handle the bust without the government's help; and Mulderig never hesitates to criticize Doyle on items ranging from trivialities like Doyle's appearance to an incident in the past where a policeman was killed and Mulderig clearly holds Doyle responsible for it; when Mulderig caustically states, "the last time you were dead certain, we had a dead cop," Doyle comes to blows with Mulderig and the two must be separated by Simonson and Russo.
Charnier soon "makes" Doyle and decides he has to be eliminated. Charnier's henchman Nicoli, who assassinated the French detective, offers to do the job and tries to kill Doyle from a rooftop with a rifle. He botches the job and a cat-and-mouse pursuit underneath the BMT West End Line begins, which eventually leads up to the car chase scene described below. The chase ends when the elevated train Nicoli has hijacked crashes into another train; when Doyle catches up with Nicoli, he shoots Nicoli in the back while he attempts to escape by running back up the stairs leading to the train platform. The car containing the drugs that Devereaux imported into the U.S. is eventually staked out by the police and impounded when some young thieves try to strip the car of its valuables. Doyle and Russo then rip the car apart in an hours-long search, before eventually finding the narcotics after the mechanic states that he has stripped everything on the car except the rocker panels.
At the film's climax, it seems like the drug deal which takes place at an abandoned factory on Ward's Island has been a major success; Boca and Weinstock's resident heroin expert tests the substance and declares it to be of top quality. In return, using an old car that Sal Boca's brother Lou picked out, the criminals stash the money in almost the same hiding place that was used on the car Devereaux brought in. The car is to be imported into France, where Charnier will then retrieve the money. Charnier and Sal Boca drive off and only moments later run into a roadblock consisting of a large force of police officers, led by Doyle. The police chase Charnier and Sal Boca back to the factory grounds, where Sal is killed during a shootout with the police and almost all of the others surrender after tear gas is fired by the police.
Charnier escapes into the warehouse and a tense sequence ensues as Doyle hunts for Charnier. Russo joins him in the search, which takes a sudden shocking turn as Doyle, trigger-happy and high on adrenaline, sees a shadowy figure in the distance and empties his revolver at it only a split-second after shouting a warning. To Russo's horror, the man Doyle kills is not Charnier, but Mulderig. Doyle seems unfazed by this and vows to capture Charnier, reloading his gun and running off into another room in the distance. The last sound heard in the film is a single gunshot.
Title cards before the closing credits note that of the people arrested and tried, only Joel Weinstock and Angie Boca got away without any prison time while the case against Weinstock was dismissed, and Angie received a suspended sentence. Alain Charnier was never found or tried in America. It also states that both Doyle and Russo were transferred out of the narcotics division.
Intercut with the car scenes underneath the elevated train is additional footage (shots facing the car, not from the driver's perspective) that was shot in Bushwick, Brooklyn, particularly when Doyle narrowly misses hitting a woman and her baby carriage. Many of the shots in the scene were "real", in that legendary stunt driver Bill Hickman, who also had a small role in the film as FBI agent Mulderig, actually drove the car at high speeds through uncontrolled traffic and red lights, with Friedkin running a camera from the backseat while wrapped in a mattress for protection. The production team received no prior permission from the city for such a dangerous stunt, but they had the creative consulting and clout provided to them by Eddie Egan and Sonny Grosso (which allowed normal protocol for location shooting like permits and scheduling to be circumvented), and the only precaution taken was to place a "gumdrop" style beacon on the car's roof and blare the horn. Other shots involved stunt drivers who were supposed to barely miss hitting the speeding car, but due to errors in timing accidental collisions occurred and were left in the final film. Friedkin said that he used Santana's song "Black Magic Woman" during editing to help shape the chase sequence; though the song does not appear in the film, "it [the chase scene] did have a sort of pre-ordained rhythm to it that came from the music.

Director Christopher Nolan used the car chase sequence as inspiration for the Batmobile chase in his 2005 film Batman Begins.
William Coke, the motorman who "collapses" at the controls (presumably from a heart attack) while Nicoli shoots and kills the conductor would actually die on the job from a real heart attack in Queens (though he was not operating a train at that time). 
Friedkin later attempted to outdo the Connection chase scene with the infamous backwards car chase on a freeway in To Live and Die in L.A..
The Pontiac Le Mans used in the chase was auctioned for an estimated $350,000 at the AFI Ceremonial ball and purchased by rapper David Banner.
In the video game Grand Theft Auto IV, there is a mission named "The Puerto Rican Connection" where the player must give chase to two Puerto Rican drug dealers in a subway train. The player gets a red car much like Hackman's in the movie and, once the train has stopped, must go on the subway platform and kill the drug dealers and their bodyguards.
The movie established the careers of both Friedkin and Hackman, and was instrumental in ushering in an era of neo-realist directors in Hollywood during the early 1970s. In an audio commentary track recorded by Friedkin for the Collector's Edition DVD release of the film, Friedkin notes that the film's documentary-like realism was the direct result of the influence of having seen Z, a Greek film. Additionally, this was the first film to show the World Trade Center: the completed North Tower and the partial completion of the South Tower are seen in the background of one scene.
The sequence on the Times Square-Grand Central shuttle took two days to shoot. Car 6609 has been preserved and is in the New York Transit Museum. It is occasionally operated on fantrips along with other preserved cars.
1973's The Seven-Ups is closely related as it stars Roy Scheider and Tony Lo Bianco, was directed by Philip D'Antoni, written by Sonny Grosso and features another famous car chase choreographed by Bill Hickman. The score for this film was also by Don Ellis.
In 1986, an NBC television movie, Popeye Doyle, starring Ed O'Neill in the title role, was also produced.