And Lovey!
I know a couple of older men (in their 80’s) are very wealthy who actually sound like a less exaggerated version of Thurston Howell. I assume it partly comes from going to certain prep schools and is partly an affectation.
Funny you should mention Thurston Howell III–I thought Jim Backus was doing his Roosevelt impersonation on Gilligan’s Island.
Billie Burke, although born American, was raised in London, and started her acting career in the West End at the age of 19 or so.
I have something of a mid-Atlantic accent, myself, having been exposed to Canadian influence when I was a child, having had a favorite teacher who was an Oxbridge man, having been in a high-school production of My Fair Lady, having been a semi-pro opera singer in my 30s and 40s, and having done a good amount of Renaissance faire in my 50s and 60s. Over and over there’s been something to tug in that direction.
This is a good video about that accent. It was taught to kids in some schools in the US, and to adults in broadcasting and acting schools. Not natural, completely acquired.
Are you thinking about Edward Everett Horton’s character, Lovett, in “Lost Horizon”?
Dan
No, Lovey Howell on Gilligan’s Island.
Lovey was actually a pet name. Her Christian name was Eunice.
Ah! My confusion stems from watching very little TV. Except old movies - add one of the very best antiques to your list - Lost Horizon, 1937.
Dan
I’ve seen it, thanks. Wonderful film! 
Yeah, no. No one speaks or has spoken with a transatlantic / midatlantic / midlantic accent naturally. Plenty of people have grown up with a lockjaw/Brahmin accent. The two sound very different.
I grew up one town over from Locust Valley in the 60s and 70s, and occasionally encountered middle-aged or older people with Lost Valley lockjaw. (Only occasionally because we didn’t travel in the same circles.) Their lower teeth remained in contact with their upper teeth throughout the entire conversation. IIRC, that’s what Backus was doing on Gilligan’s Island.
The midlantic accent was entirely invented by the movie studios in the 1940s (I think). People had to learn that accent, and they were free to move their jaws up and down when they spoke with it.
Actually, it just happened that to become a star, you had to learn to effect the accent. So pretty much everyone in the movies from the beginning of the talkies spoke that way. You’d even have dirt-poor characters speak that way, if they were the leads. It’s kind of like perfect, white teeth on homeless people in 21st century TV.
Irene Dunne had grown up in Kentucky, although she’d also lived a bit in a small town in Indiana, where people sounded pretty much like they did in Kentucky.
She had a perfect trans-Atlantic accent when she needed one, but there were a few times when she played a character who was trying to pretend to be either southern or from a lower class, and not necessarily doing it perfectly, and she pulled off an hysterical mix of the two accents.