Yeah that entails a lot, and either way he was now going to pay child support just by marrying their mother.

Carol's first husband must have been a deadbeat because Mike adopted the girls and changed their last names shortly after he married Carol. It seldomly happens that fast in real life.

Some fathers are @ssholes, sadly. I guess Carol's husband was one of them.

Hmmm I remember seeing the pilot and subsequent first season eps in reruns growing up. I think the implication was that the dad had died. Carol had moved back in with her parents, along with her three kids. And numerous first season eps mentioned Marcia, Jan and Cindy talking about their "new daddy." I think that implies the first/original dad died. Regardless of whether parents divorce, kids usually remain pretty loyal to their biological parents. The term "new daddy" kind of implies that the original dad died and Mike is stepping up and taking his place, not that original dad still out there.

Yeah, Schwarts is adamant that Carol divorced her first husband, and as far as anyone knows, he has been a complete non-entity in the girls lives since perhaps even before Carol got married.
I don't know for certain, but Mr. Martin may very well be TV's first dead-beat dad.
Now, due to the times, the show couldn't come right out and say Carol was divorced, so there could never be any episodes featuring the dad. It probably would have been "too heavy" for the show, anyways.
That could have made a shocking ending to "A Very Brady Christmas", when Santa removes his beard and it's Mr. Martin underneath, lol.

Well, what was the bio father's name. Maybe coincidentally, it was also Mike and Marcia named it after both her fathers.