In November 1981, Robert Wagner (Jonathan Hart) and Stefanie Powers (Jennifer Hart) each lost their significant others: actress Natalie Wood and actor William Holden, respectively.
The Harts' dog, Freeway, was so named because he was found by the side of a freeway.
The dog who portrayed Freeway, was found in an animal shelter and was named "Charlie Gray". On the set, he was especially close to Stefanie Powers. His offspring portrayed "Junior" in the later Hart to Hart movies.
The last Hart to Hart movie aired on Aug 25, 1996, exactly seventeen years, to the date, of the airing of the pilot.
The Harts' ranch-style house originally belonged to Dick Powell, an old friend of Robert Wagner, and was situated in Mandeville Canyon, Los Angeles, California.
Actresses considered for the role of Jennifer Hart included Suzanne Pleshette and Lindsay Wagner.
In season five, episode twenty-one, "Always, Elizabeth", June Allyson guest starred. She had previously lived in the Hart house with Dick Powell and their children, Pam and Rick.
The initial choice for the role of Jonathan Hart was Cary Grant. However, Grant (who was seventy-five years old at the time) had effectively retired from acting several years earlier.
The exterior shots of the Harts' lavish ranch-style house was used as the exterior of Lora Meredith's house in Imitation of Life (1959).
In the German dub, the Harts' dog Freeway is named Friedwart, a medieval name meaning "guardian of peace".
On November 29 1981, when this show was still on the air, Natalie Wood, husband Robert Wagner, and friend Christopher Walken took a late night boat ride, during which Wood drowned in the Pacific Ocean. The death was originally ruled an accident, but now the investigation has been re-opened.
Sidney Sheldon had originally written a script for CBS called "Double Twist", about a married couple who were also spies. Aaron Spelling and Leonard Goldberg decided to update the idea for a potential television series. They asked Tom Mankiewicz to update the script, to make it more contemporary, and viable for a potential weekly series. Mankiewicz re-worked Sheldon's original script, and it was renamed "Hart To Hart". Mankiewicz also made his directorial debut with the pilot episode as planned, and remained a Creative Consultant on the series.
In season four, episode nineteen, "A Change of Hart", the Hart address is listed as 3100 Willow Pond Dr. Bel Air, California, 29042. However, that ZIP code is actually located in Denmark, South Carolina. In season five, episode twenty-one, "Always, Elizabeth", the Hart's address is listed as 3100 Willow Pond Dr. Beverly Hills, California, 90047.
Lionel Stander (Max) played opposite Robert Wagner's Alexander Mundy on It Takes a Thief (1968), season three, episode nine, "The King of Thieves". Stander's character in that episode was also named Max.