I was very young when this show was aired; but I do recall that Mannix had a black wife/woman. At the time I thought "whoa !" but then, "why not ?". I was expecting some confirmation of their relationship in the triva or other comments here. But no matter, 'cause you can call me Mannix (hehe-hint hint).
Although not married you had the feeling that there was something more than just business between them. I don't think the series ever elaborated on that aspect of their relationship but I think most Mannix fans believed there was something going on between them.
The "sexual-tension" between a male boss and his secretary is a standard plot device that has been used for ages. The fact that Peggy was black just forshadowed that attitudes toward inter-racial dating were about to change. This happened around the same time as the famous kiss between Capt. Kirk and Uhura on "Star Trek".
Peggy was the widow of Joe's friend Marcus who was a police officer killed in the line of duty. I will agree that there was some flirtation that did indeed exist but I think the respect Joe had for Peggy and her son was far greater.
They were not a couple, but the addition of Gail Fisher to the cast helped boost ratings. The black audience started watching in large numbers helping "Mannix" move up in the ratings.
It probably also didn't hurt that, in addition to being a beautiful woman, Gail Fisher was a fine actress who had great on-screen chemistry with Mike Connors. Although her role was never as large as it might have been, there were a number of episodes that focused on her and her son, as well as episodes where, e.g., she would assist Joe in working on a case. This took her role beyond merely being a stick-figure character who just answered the phone and gave Joe messages about whose call he was supposed to return.
In many of Sidney Potiers films he is cast against nuns and blind girls. I understand that that was done on purpose to keep the lead actor from getting into any romantic entanglement on screen. The secretary's job made Peggy more of a person than some "old flame" or some chic he dated. In retrospect it was a pioneering role.
I remember two things. Mad Magazine did a pardody of the show, where Mannix is too stupid to see that Peggy wants to marry him. Also Rex Reed had a problem. He was critical and I remember on Mike Douglas, when pressured to give the reasons why, Reed claimed Mannix didn't eat lunch out of papr bags like the rest of us and was too flashy. After that I saw a show where he did bring a brown bagged lunch.
As I understand it, Peggy, the secretary was the widow of a slain police officer. (I get the impression she helps Mannix because she hates criminals and Mannix looks like he will get killed fighting crime also, and so she tries to stop this)
Lot of people thought something going on between Perry Mason and Della Street too. He does seem to buy her a lot of dinners, course I'm not sure how many she actually got to finish.
I was an adolescent and pre-teen when I fell in love with this series and even back then? I could tell there was a chemistry between Peggy and Mannix!!
It was the sexual fantasy of the white writers on the show "inferred"...having the lovely and helpless black secretary being kidnapped and (most likely) sexually assaulted while being held captive and then she being so overwhelmed by Mannix's bravery when he eventually rescues her she "opens up" and gives him off the chain sexual pleasures...for many years this "notion" played out in the twisted imaginations of writers in Hollywood...