Frank Sinatra played Detective Joe Leland from the novel "The Detective" by Roderick Thorp. Thorp wrote a sequel ("Nothing Lasts Forever") in which Leland is trapped in a Claxxon Oil Corporation skyscraper after it's taken by German terrorists, and must rescue his daughter and grandchildren. Twenty years later the novel was filmed with some changes: the daughter became his wife, Claxxon became the Nakatomi Corporation, and Joe Leland's name was changed to John McClane. The film was released under the title Die Hard (1988). Because of a clause in Sinatra's contract for "The Detective", which gave him the right to reprise his role in a sequel, he was actually the first person offered the McClane role, even though he was 73 years old at the time. Also, coincidentally, Bruce Willis (who played McClane) made his movie debut in The First Deadly Sin (1980), walking out of a bar as Sinatra walked into it. Additionally, Lloyd Bochner played Dr. Wendell Roberts in this movie. His son, Hart Bochner, played Harry Ellis in Die Hard (1988).

Frank Sinatra was supposed to co-star with his wife Mia Farrow in this film, but a film Farrow was working on was running behind schedule, so she refused. Sinatra got so mad, he made the film without her (casting Jacqueline Bisset in the role instead) and served her divorce papers on the set of that film, Rosemary's Baby (1968). When both films were released and Rosemary's Baby performed better at the box office over The Detective, according to Robert Evans's autobiography The Kid Stays in the Picture, a particularly embittered and vindictive Farrow wanted to get back at Sinatra by having Evans post an advertisement in Variety comparing the two films' box office and popularity, but Evans wouldn't.

Frank Sinatra and Ralph Meeker, who appeared in this movie, played the same character (Francis X. "Iron Balls" Delaney) in two other movies. Meeker played him in uniform in The Anderson Tapes (1971). Sinatra played him in plainclothes as a detective in The First Deadly Sin (1980).

Mark Robson was originally set to direct, but Frank Sinatra preferred Gordon Douglas, with whom he had made four previous films.

Film debut of Tom Atkins.

Suzanne Pleshette and Barbara Parkins auditioned for the role of Norma McIver.

Robert Evans had acquired the rights to "The Detective" and was supposed to produce the movie for 20th Century Fox, which had signed him to a contract (he had earlier worked for Fox as an actor). When Charlie Bluhdorn, the owner of Paramount Pictures, offered Evans a job as the head of European production at Paramount Pictures, Evans had to surrender the project to get out of his Fox contract.

The Detective, along with Klute (1971), represents one of the last "old school" police movies, with its distinctively confined and moody feel and it could easily have been mistaken for having been filmed in the 1950s. A step forward in crime dramas was in Bullitt (1968), which came out the same year as The Detective. Another step forward was in The French Connection (1971), but like Bullitt, it still had a closed-in moody feel of the older films. Considering that The Detective came out the same year as the revolutionary leap forward in sci-fi via 2001: A Space Odyssey (1968) and only two years before the revolutionary leap in war films represented by Patton (1970), the old style of detective films was clearly on its last legs. When Klute (1971) came out three years later, it put a definitive end to the late noir style.

The brief establishing shot of Leland at the football game (at about 22 minutes in) was shot at an actual NFL game. It was the Green Bay Packers at the New York Giants, played on October 22, 1967. Visible on the field are, for the Packers: kicker Don Chandler (#34), offensive lineman Bob Hyland (#50) and linebacker Jim Flanigan (#55); for the Giants: defensive end Randy Staten (#83) and guard Bookie Bolin (#63). The Packers won the game 48-21.

Tom Atkins has a small role as a police officer, Joe Santos appears unbilled as a reporter. Both actors would later play police colleagues on The Rockford Files.

This film came out two years before Jack Klugman suddenly become well known to the public by starring in the TV comedy The Odd Couple (1970) and eight years before he would be recognized as a serious actor by starring in Quincy M.E. (1976). Having grown up on the rough streets of Philadelphia, which included work as a street pedlar, Sinatra had a great deal of respect for him and it was during this period after the Rat Pack that Klugman would become a member of Sinatra's outer circle and being invited to his parties and events in Palm Springs. His performance in this film doing forensic accountant work could be seen as a slam-dunk audition for his role as Medical Examiner R. Quincy.

Robert Duvall and Jacqueline Bissett both appeared in another police procedural released the same year as this one... Bullitt.

The residential set designs represent what was a thankfully brief early-sixties trend in which all of the walls, doors and cabinetry were painted the same flat color or, as seen in Leland's apartment, included wall paper in the same color and value as the painted elements. The most extreme example of this style in other films can be seen in the interiors of Please Don't Eat the Daisies (1965), which had been released three years earlier. This style can be seen prominently in the office of Dr. Roberts.

When MacIver enters the gay bar the background music is an instrumental of the song Laura by Johnny Mercer. It is about romance with a passing stranger and which is more of a yearning than a reality. MacIver is in pursuit of the same elusive and non-permanent attraction.

Lee Remick was 20 years younger than Frank Sinatra.

In an interesting touch, during the flashbacks, whenever Joe and Karen have a conversation, each looks directly at the camera, as if we're watching from the other's point-of-view. This approach was tried with several films during that period but abandoned because it made audiences uncomfortable (men in particular were not into gazing into Sinatra's eyes during a personal moment).

The entire film treats and depicts gay men as deviants beyond redemption and the cops as being justified in beating them at will, as well as scoffing at the concept of bisexuality. As nasty as it was in 1968, it's even more objectionable today. (Ironically, the evil cabal at the heart of the mystery is called Rainbow.)

Film debut of Don Fellows.

Frank Sinatra and Richard Krisher both play in Gordon Douglas' previous movie "Tony Rome" (1967)

Farrell was a contemporary of Joe's father, implying he's at least 20 years older. In real life, MacMahon and Sinatra were only nine years apart in age.

Sinatra made his career, and established his personal style in the 40s. By the time this film came out hippies and psychedelia had revolutionized popular culture, and the Beatles' "Sgt. Pepper's Lonely Hearts Club Band" was revolutionizing music. The use of a sitar in the opening theme song to this film is one of the few meager attempts at modernizing his outdated appeal. This was also the era when Sinatra began combing his thinning hairline (toupee) more towards the front.