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 one hundred percent", but Buck Henry decided 99 sounded more feminine.
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. Adams chose the ownership deal and never regretted it considering the series' durable popularity in syndication gave him a regular income even as he struggled with being typecast by it.
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.
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 played Larabee, was Don Adams' cousin.
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.
Barbara Feldon was initially reluctant to commit to the series. She refused to sign a traditional five-year contract, and signed for only four episodes. After filming the pilot, she agreed to commit to the series, but signed a three-year contract. After the third season, she agreed to sign for two more years.
According to Buck Henry, Talent Associates Producer Daniel Melnick came up with the basic idea for the show. Melnick told Mel Brooks and Henry that he wanted a character based on the two most popular film franchises at the time: the James Bond and The Pink Panther film franchises.
The creators first submitted the series idea to ABC, with Tom Poston as Agent 86. When ABC rejected it, they brought it to NBC, which would only finance a pilot if the role was re-cast with Don Adams, whom they had under contract.
Agent 99's real name is never revealed, not even when she marries Max, 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 1965, before the show went on the air, the publicity firm of Rogers & Cowan sent 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 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 1960s Ferrari 250 GT Cabriolet.
This show was the principal inspiration for Inspector Gadget (1983), of which the main character was voiced by Don Adams, and was also a bumbling and inept Agent. Also, some of the lines from this show, such as "Would you believe..." and "Sorry about that, Chief", were uttered by Gadget on occasions.
During development, Mel Brooks and Buck Henry discussed many possible names for their super-spy, including Lance, Dagger, and Bounty Hunter. Brooks came up with Smart - initially Raymond Smart, later Maxwell. He said in later interviews the name was derived from his father Max, as was the case with Max Bialystock in The Producers (1967).