Mannix was one of the last dramatic series to be produced by Desilu before the company was absorbed by Paramount, which occurred following the first season. Amazingly, a format change enabled after that first season may be what kept the series going for as long as it did.
Originally, Joe Mannix (Mike Connors) was an operative for Intertect, Ltd., a security/detective agency that had a larger-than-normal office and relied on computer technology. Desilu co-founder Lucille Ball felt that the computers were a little too high-tech, and so, Intertect and its frontman, Lew Wickersham (Joseph Campanella, ex-Batman) were phased out. Campanella left the show to move over to NBC's The Bold Ones: The New Lawyers. Mannix would operate on his own for the remaining seven seasons, gaining a secretary, Peggy Fair (Gail Fisher), and subsequent stories would deal with relevant social issues of the times.
Creators Richard Levinson & William Link would leave Paramount, and develop Columbo, Ellery Queen, & Murder, She Wrote for Universal during the 70's & 80's, the latter series bringing them back to CBS. Interestingly, more than 20 years after Mannix ended, Mike Connors would reprise the role in an episode of Dick Van Dyke's Diagnosis: Murder, which served as a sequel to a Mannix episode.
Here's the open for the first season:
I believe that is Hank Simms, who'd later be the announcer for Quinn Martin's family of crime dramas, doing the voiceover for a sponsor cue.
The series had been seen on cable on TV Land in recent years, and would be a good candidate to move on to Me-TV down the road, since that network also has the rights to the other Desilu-Paramount series from the era, Star Trek & Mission: Impossible.