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

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).

Film debut of Barbara Harris.

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.

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

Included among the American Film Institute's 2000 list of the 500 movies nominated for the Top 100 Funniest American Movies.

After this movie Barbara Harris and William Daniels appeared together in the Broadway hit "On a Clear Day You Can See Forever" running from Opening Date: Oct 17, 1965 Closing Date: Jun 11, 1966.

Martin Balsam's Oscar winning performance in this film is his only Academy Award nomination.

Michael Gordon was the original director but he left before shooting began.

Martin Balsam and Jason Robards would later appear together again in "All the President's Men" (1976).