Though the chief's first name was Thaddeus, he also used the cover name Harold Clark in the final two seasons.
The red convertible driven by Don Adams in the opening credits for the first two seasons is a 1965 Sunbeam Tiger Mark I, which came standard with a Ford 260 V8 engine. The car was used in several episodes throughout the first four seasons, though sometimes substituted by a very similar-looking Sunbeam Alpine with Tiger badging. A Volkswagen Karmann Ghia was used in the opening credits for the third and fourth seasons but never used in any episode. For the fifth season, the show featured a 1969 Opel GT used in the opening credits as well as the episodes for that season. Only used in the pilot episode was an early-'60s Ferrari 250 GT cabriolet.
It is rumored that Agent 99 was originally supposed to be named Agent 69 but NBC censors deemed it to be too "sexually suggestive". According to Barbara Feldon, this is not true. Her character was originally to be named Agent 100 "because she was 100 percent" but Buck Henry decided 99 sounded more feminine.
Agent 99's real name is never revealed, not even when she marries Smart, after which was is occasionally referred to as Mrs. Smart. In one episode her name was said to be Susan Hilton, however she later recanted and claimed it was an alias.
In the final season, Max and 99 have twins, a boy and a girl. The boy is likely the individual who grows up to be Zach Smart in the later revival "Get Smart" (1995).
Barbara Feldon was two inches taller than Don Adams. In order to make it appear that Adams was taller than Feldon, he would either stand on a small platform or Feldon would stoop down. Also, for most of the show's run, Feldon wore mostly flat shoes and very rarely wore high heels.
In an episode of the final season where Larabee has a much bigger role than in previous years, Max says to the Chief after a typical Larabee blunder, "People have often asked me if Larabee and I are related." Robert Karvelas who plays Larabee was Don Adams' cousin.
King Moody auditioned for the role of Hymie. He later appeared as a double agent in an episode before being cast in the role of Starker.
During the 4th season the Chief had a 1968 Ford Shelby Mustang convertible that Max sometimes drove.
When Don Adams was negotiating his salary, he had his choice between more money per week and no ownership stake in the show or less money per week and part ownership. He chose the ownership deal and never regretted it.
Agent 99 was named "most stylish secret agent" at the 2004 TV Land awards.
When asked how he developed his trademark voice for Agent 86, Don Adams said that it came from watching "The Thin Man" movies starring William Powell and Myrna Loy. After hearing Powell repeatedly say, "Asta! Asta!" he decided to imitate his voice for the character of Maxwell Smart.
CONTROL and KAOS were supposed to be acronyms, but Mel Brooks and Buck Henry never came up with anything for them to stand for.
The Chief, played by Edward Platt, has an assistant named Larrabee. Larrabee is the name of Platt's character in North by Northwest (1959).
In 1965, before the show went on the air, the publicity firm of Rogers & Cowan send a series of five sealed envelopes to America's top television critics. Each was stamped 'Top Secret' and contained a vital message from Maxwell Smart. The first read: "Sssh!". The second provided instructions for deciphering the enclosed message embossed on an enclosed pin. The third required a 'cigarette lighter or matches used judiciously' to reveal the invisible message. The fourth read: 'Je was linker om iemand anders te nemen om je dit voor te laten lezen" which is Dutch for "You would have been smarter to ask someone else to read this aloud for you". The fifth included a photograph of Max, 99 and Fang with their eyes blocked out with black rectangles and a caption that read 'A scene from ____ at ____ on ____".
The series was probably a spoof of Quality Comics's old T-MAN comic book series, which featured a main character called Peter Trask (the T-Man of the title), as the format of the show and the comic are very similar - both Pete and Max dealt with counter-espionage, both refer to their superior officers as "the Chief," and both have three-syllable names.