Director James Sheldon cast Tony Randall in what was supposed to be a small role in a single episode. The producer was so impressed with Randall's work that the role was expanded, and he became a regular on the show.
The role of the gym teacher (played in the series by Jack Warden ) was played in the pilot by an uncredited Walter Matthau.
'Mister Peepers' was originally a summer series only for NBC and wasn't on the fall schedule for 1952. Doc Corkle (1952) was originally on NBC's schedule, but was canceled after only 3 episodes. 'Mister Peepers', which had scored well with viewers and critics during its summer run, replaced 'Doc Corkle' on the schedule.
Wally Cox married girlfriend Marilyn Gennaro in real life around the same time as the Peepers wedding was taped in May of 1954.
After it was announced that Nancy Peepers would have a baby, American Character dolls decided to make and sell a "Peepers Baby" doll in 1955. Getting info from producers that the Peepers baby was to be a girl (polling data indicated viewers would prefer a girl), the doll was still sold after "Mister Peepers" was canceled. In 1956 it was still sold, but other a new name, not Peepers Baby.