Robin Masters' voice was provided by Orson Welles in five episodes, with a final appearance voiced by a different actor, Red Crandell.

In the pilot episode, Rick's bar was originally called 'Rick's Cafe Americain', inspired by Casablanca, complete with 1930s attire, but executives felt audiences wouldn't connect with this concept, so it was changed to the King Kamehameha Club.

At the end of season seven, Magnum was originally supposed to be killed off to end the series, but fan outcry led to an eighth season being produced to bring him back and give the show a proper conclusion.

Tom Selleck's contract commitment to Magnum, P.I. famously cost him the role of Indiana Jones in Raiders of the Lost Ark, which went to Harrison Ford instead.

Due to the 1980 AFTRA/Screen Actors Guild strike, production of Magnum, P.I. was delayed until December 1980, which ironically would have allowed Selleck's schedule to accommodate playing Indiana Jones after all.

Many indoor scenes of Magnum, P.I. were filmed on the old Hawaii Five-O soundstage, since the network didn't want its Hawaiian production facilities to go unused after that show ended.

The producers tried unsuccessfully to convince Jack Lord to make a cameo as his Hawaii Five-O character, though characters referenced Five-O by name in early episodes, such as Magnum posing as 'McGarrett of Five-O' over the radio.

The real Robin's Nest estate, known as 'Pahonu' or 'The Anderson Estate' on Oahu's east shore, was demolished in April 2018 by its new owner, a friend of former President Barack Obama, after being sold for $8.7 million in 2015.

The original pilot theme was a jazzy piece by Ian Freebairn-Smith, later replaced starting with Episode 12 by the more famous up-tempo theme by Mike Post and Pete Carpenter featuring guitar by Larry Carlton; a single version reached No. 25 on the Billboard Hot 100 in 1982.

The vintage Bentley 3.5 Litre driven by Magnum and Higgins in England during the Season 6 opener had previously been driven by James Bond in the unofficial 1983 film Never Say Never Again.

A crossover script was written in 1992, three years after Magnum ended, for Donald P. Bellisario's Quantum Leap, in which Dr. Sam Beckett would leap into Thomas Magnum's body; test footage exists of Scott Bakula recreating Selleck's famous eyebrow-raise shot from the opening credits.

In September 2017, a group of 45 men dressed as Thomas Magnum were ejected from a Detroit Tigers game for smoking and catcalling, drawing international media attention.

In October 2013, Tom Selleck revealed that author Tom Clancy had planned to write a screenplay for a Magnum, P.I. film in the 1990s with Universal Pictures interested, but the project never materialized.