Quinn Martin, after dealing exclusively with ABC for a decade, turned to CBS for his next project, Cannon, which lasted 5 years (1971-6). The series also reunited Martin with star William Conrad, who'd previously narrated The Fugitive. In Conrad's case, after forging a career in radio (Gunsmoke, in particular) and in Westerns, this was his first starring role on TV, and the first of three such gigs.
Frank Cannon was a former police detective who left the force after the deaths of his wife & son, a case that was left unsolved until the final season. Cannon then became a private detective, and, in standard fare of the period, often took a few lumps before finally closing the case.
Four years after Cannon ended, CBS commissioned a TV-movie, "The Return of Frank Cannon", which brought closure to the series. In between, Conrad went back in the announcer's booth and narrated another QM entry, Tales of the Unexpected, for NBC, and did the season 1 voice-over for Buck Rogers in the 25th Century, also for NBC.
Here's the intro: