A third-season appearance of Harry Mudd was planned but axed due to the producers' desire to move away from comedy episodes. However, Roger C. Carmel would reprise the role of Mudd as a cartoon voice in Mudd's Passion (1973). Mudd was considered for a return during the Star Trek movies in the 1980s, but Carmel's failing health nixed that.
During the filming, casting director Joseph D'Agosta was in a panic because he needed at least two female identical twins and couldn't find any suitable for the show. Then one night while driving home he saw Alyce Andrece and Rhae Andrece walking down a street. D'Agosta literally pulled up beside them, jumped out of his car and told them that they were going to be on television! (In some tellings of the story Gene Rodenberry is substituted for D'Agosta, but Steven Whitfield's "The Making Of Star Trek" confirms it was D'Agosta.)
With the exception of those actors who played members of the Enterprise crew, Roger C. Carmel was the only actor to play the same character in more than one episode of Star Trek the Original Series.
According to Walter Koenig, NBC considered making a Harry Mudd spin-off show after the success of "I, Mudd." They assigned Gene Roddenberry to develop the idea, but being busy with Star Trek and other projects, he didn't have time for it, and the series was never conceived.