Trivia

According to William Daniels' memoir, several months after production on the film concluded, he attended a private screening of an initial cut that contained no location filming, no marching band music (as was featured in the play), and Paul Richards in the role of Leo Herman, not Gene Saks who had successfully played it on stage but was originally unavailable for filming (ironically, Richards had taken over the part of Leo from Saks in the original Broadway production). This early cut proved to be such a disappointment to the filmmakers, Herb Gardner decided to relinquish his screenwriting fee in exchange for permission from the producers to rewrite several scenes, hire the now-available Saks as a substitute for Richards, shoot a number of exterior scenes on location, and extensively re-edit the film into its final version.

The original Broadway production of "A Thousand Clowns" by Herb Gardner opened at the Eugene O'Neill Theater in New York on April 5, 1962, ran for 428 performances and was nominated for the 1963 Tony Award for Best Play. Jason Robards, Gene Saks, William Daniels and Barry Gordon recreated their stage roles in the filmed production. Gordon was nominated for the 1963 Tony Award for Best Featured Actor in a Play. Sandy Dennis who did not recreate her stage role, won the 1963 Tony Award for Best Featured Actress in a Play.

Two of the names Nick adopted temporarily are those of specific real people. Dr. Morris Fishbein was the very controversial editor of the Journal of the American Medical Association (JAMA) from 1924 to 1950. In 1947 Time Magazine described him as "the nation's most ubiquitous, the most widely maligned, and perhaps most influential medico." Rafael Sabatini was an author, and a few of his most popular novels were adapted for film. These films include Captain Blood (1935) and Scaramouche (1952).

This movie features a scene where Jason Robards walks in front of the Lincoln Center for the Performing Arts during its construction.

At a March 5, 2022 screening of West Side Story (2021), Steven Spielberg revealed that he and his production team studied A Thousand Clowns for reference, to see what Lincoln Center looked like during construction. The performing arts center can be viewed mid-construction in the film.