Mike Connors claimed that CBS canceled the series due to Paramount Television selling reruns of it to ABC, its competitor, that doesn’t seem to make a lot of sense. Mannix was the #20 series according to the Nielsen ratings of 1975 (its last year in production), Connors was a popular, but older star who was unlikely to leave to try his hand at movie fame; and the series was “replaced” by moving the police procedural Kojak into its place rather than using a new program.

Connors was a great actor and his show, Mannix, stayed good through all eight seasons. A lot of shows peter out near the last season but I think Mannix was just as good in S8 as in the earlier seasons.