From TV Line: "TV Vet Joseph Campanella, of Mannix and One Day at a Time, Dead at 93"
Commander Benson details his character's value to Mannix, at least during the first season, here: "Mann(ix) Versus Machine"
and here are two more threads about Mannix:
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Here's a lengthy overview of Mannix from TV Club: "The Long-Running Private Eye Series Mannix was Brutal, Stylish Comfort Food"