CBS was going to cancel the series after the first season. Lucille Ball used her power and influence to convince them to renew it for another season with the assurance that changes would be made. In the second season, Mannix was changed into a more hard-boiled independent private detective. The changes worked, and the series became a big hit running for eight seasons.
Mike Connors complained that this show was not very good at showing the consequences of violence. He said Joe Mannix would get thrown down a flight of stairs in one scene, and then appear without a scratch in the next.
This was the last television series to be produced by Lucille Ball's production company, "Desilu."
The role of Peggy Fair was intended for Nichelle Nichols, but she was forced to withdraw due to her commitment to Star Trek (1966).
Mike Connors reprised his role as Mannix on "Diagnosis Murder" (1993) season four, episode seventeen, "Hard-Boiled Murder." It was a sequel to season seven, episode four, "Little Girl Lost" of this show. Pernell Roberts and Julie Adams, who appeared as guest stars in the "Mannix" (1967) episode, also reprised their roles on "Diagnosis Murder" (1993).