When Harry meets Uhuru leader, Mustapha (played by Albert Popwell), he asks "Where do I know you from?" - Popwell played the bank robber in "Dirty Harry" to whom Harry first asked "do you feel lucky?", and in fact appeared, playing different characters, in all but the last Dirty Harry film, "The Dead Pool".
Clint Eastwood was originally going to direct this film himself. After taking over as director of The Outlaw Josey Wales (1976) though, he found himself having to supervise the edit of that film and lacked time to fully prepare himself to both act and direct in another film so soon. He therefore promoted James Fargo, who would have been this film's assistant director, to full director. Eastwood would instead direct the next Dirty Harry film, Sudden Impact (1983).
According to Clint Eastwood biographer Marc Eliot, Stirling Silliphant suggested Tyne Daly for her role, but the actress turned down the script three times before finally deciding on doing it. Daly also suggested that the romantic attachment between her character and Callahan be omitted. Eastwood agreed.
According to director James Fargo, at Clint Eastwood's request, an improvised bar scene with Tyne Daly and himself drinking in a bar was set up. After drinking four quarts of beer between the two, they managed to get one good line by her saying she had to go pee, which she did, but the scene overall didn't work and was deleted.
As with other movies in the "Dirty Harry"-series, Callahan makes use of a repeated catchphrase. In Dirty Harry (1971) there was the "Do I feel lucky?" speech, in Magnum Force (1973) there was "A man's got to know his limitations", in Sudden Impact (1983) there was "Go ahead; make my day" and in The Dead Pool (1988) there was "You're shit out of luck". In this film, Callahan repeats the phrase "You can count on that/it" twice and the word "Marvelous" (sarcastically) four times.
The two militant organizations depicted in the film - the People's Revolutionary Strike Force and Uhuru - were modeled after two real-life militant groups, the Symbionese Liberation Army (which kidnapped Patricia Hearst) and the Black Panther Party.
In 1980 a writer sued Clint Eastwood for plagiarism, accusing him of taking the title of the film from one of his works. Eastwood maintained that he was inspired by the Humphrey Bogart film The Enforcer (1951) (which was also owned by Warner Bros). The case was dismissed.
According to 'American Rebel', "The Enforcer" grossed a phenomenal $60 million in its initial domestic release, doubling that overseas and making the film Clint Eastwood's biggest grosser up to that time.
When Harry is about to enter the liquor store, his partner DiGeorgiou asks if he needs help but Harry responds that he 'may need to move fast and doesn't need too much linguini slowing him down'. This is a reference to Dirty Harry (1971) when DiGeorgious cannot climb over the fence with Harry to get into the football stadium due to his weight, with the excuse "Too much Linguini".
Two hoods' names refer to live people. "Lalo" is the first name of the composer of the other "Dirty Harry" movie soundtracks: Lalo Schifrin. "Buchinski" is the real last name of Clint Eastwood's "competitor" in the action genre, actor Charles Bronson.
Tyne Daly says that the only time she ever saw Clint Eastwood get angry was when they were filming the climactic scene between her character and his and a production manager was hurrying the crew to wrap it up. Eastwood snapped that they had made it this far and that there was no need to get cheap now. His wishes were followed and they kept shooting until Eastwood was happy with the shot.
The PRSF uses the former Alcatraz maximum security prison to hold the Mayor of San Francisco hostage; at the time of the film's release, Alcatraz was listed in the National Register of Historic Places in 1976. Some of the buildings seen in the film were destroyed during the Native American occupation between 1969 - 1971.
At one point during the movie, a phone call is made from outside Candlestick Park (where the Giants are playing) to a pay phone that is in front of a bridge and a warehouse. The warehouse has since been torn down, replaced by AT&T Park (the home stadium of today's Giants). Fans walking to AT&T Park from the south cross the bridge to get there from the parking lots.
During the firearms heist (known as Hamilton Firearms in the film) in a warehouse dock, the firearms heisted are real-life infantry weapons in service with the United States armed forces. The firearms include M16A1 assault rifles, Armalite AR18 assault rifles (the AR18 was not adopted as a standard service assault rifle - it was later adopted by the Provisional IRA in Northern Ireland - the villain Tex is seen firing an AR18 during the Alcatraz shootout), and the M72 anti-tank missile, known as the Light Anti-Tank Weapon; in the film, the M72 was called the LAW Rocket. Both the M16A1 and the M72 were used in the Vietnam War.
After Pauline Kael gave a bad review of the film, Clint Eastwood asked a psychiatrist to do an analysis of her from her reviews of his past work, which he had memorized verbatim. It concluded that Kael was actually physically attracted to Clint and because she couldn't have him she hated him. Therefore, it was some sort of vengeance according to Clint. [Per Sondra Locke's autobiography]
One of two occasions where a Dirty Harry sequel and a western directed by and starring Clint Eastwood were released the same year. The Outlaw Josey Wales was released over the summer with the Enforcer released that winter. Three years earlier, the Eastwood western High Plains Drifter was released in the summer with the Dirty Harry sequel Magnum Force released that winter.
Capt McKay is played by Bradford Dillman. He would later play Capt Briggs in the next Dirty Harry movie Sudden Impact (1983). These are the only two Dirty Harry movies he's in, but a Lt. Briggs was played by Hal Holbrook in Magnum Force (1973). No word if the two Briggs' are the same character or related.
The trivia item below may give away important plot points.
The cell the kidnapped mayor is held in, is located on A - Block inside Alcatraz prison. This block of cells (there are / were four blocks, A, B, C and D) was utilized by the United States Army when Alcatraz was a military prison, but A - Block was not upgraded when Alcatraz was converted into a a federal prison a feed was rarely used.