During the 1972-73 season Mannix's car (a 1973 Plymouth Cuda convertible) were actually three 1971 models updated (by changing the grill/headlights, hood, and taillights panel) to look like a 1973 Cuda, as Plymouth no longer made the Cuda as a convertible, and driving a convertible had been a Mannix trademark since the show began. Mannix began to drive the Challenger, which also went out of production.
An episode of the series Diagnosis Murder (1993) called "Hard-Boiled Murder" (episode # 4.17), was actually a sequel to the "Little Girl Lost" (episode # 7.4) episode of this series. Many of the same guest stars appeared in both episodes.
Star Mike Connors complained that this show was not very good at showing the consequences of violence. He said Joe Mannix would get thrown down a flight of stairs in one scene and then appear without a scratch in the next.
In the first season, Mannix worked for a company named Intertect. While the company relied on high-tech methods to solve crimes, Mannix used old-fashioned smarts and intuition. After the first season the producers decided to dump the "new vs. old" conflict and put Mannix into private practice.
Mannix was of Armenian descent and spoke fluent Armenian and French. In the opening credits, the screen that says "Mike Connors is" features the colors (red, blue, orange) and shape (3 rectangles) of the Armenian flag.
The olive-colored books with black and red trim seen behind Mannix's and occasionally behind Peggy's desks are actually law books often seen in attorneys' offices, which contain sequential reported appeals court decisions. As it would be unlikely for a real-life private investigator to have law books in his office, they were probably chosen simply because they were an attractive way to fill up the bookshelves, and because they could be acquired somewhat inexpensively from a bookstore or a retiring attorney.