It does sound like a “Get Smart” kind of thing to say.
The show I remember using it was the 1985 HBO/CBC mockumentary “The Canadian Conspiracy”.
“Lorne Greene…green card…coincidence?”
“Lorne Greene…Lorne Michaels…coincidence?”
Coincidence? I think NOT!
Coincidence?
Oh, sure. Most definitely!
Newman Day, celebrated at some colleges since the 1970s, is based on an apocryphal quote by Paul Newman “24 hours in a day, 24 beers in a case. Coincidence? I think not.” I didn’t find any reference to when he supposedly said this, but it would put the first date for this appearing at least in the 1970s and possibly earlier.
The TV Series “In Search Of …” was narrated by Leonard Nimoy and each episode investigated some form of woo. Nimoy would always describe some circumstance and then say “coincidence?”. He always left it at that because whatever coincidence he’d just described was obviously air tight evidence of whatever nutty idea this week’s episode was focusing on. I think he said this many times each episode or at least several times in the promo that was shown in commercial breaks in week before the show. There’s an example of Nimoy saying “coincidence” about 51 seconds into this.
This was in 1977 if I can believe youtube and I always assumed the “I think not” trope was making fun of “In Search Of …”. I’m pretty sure I started hearing “I think not” a few years after that.
Yeah, I can still remember this guy, he had a whole routine built around it, increasingly more dramatic every time he said it.
I’ll add another vote for this stand-up comedian whose name none of us can remember. (And I’ve been Googling all afternoon. Can’t find him.)
I remember his bit well. For a few weeks it was a catch phrase among me and my group of high school friends. It can be made to fit into almost any conversation, at some point.
Of course it’s possible this comedian was echoing an older TV show, like In Search Of or something similar, but I don’t remember the phrase being nearly so famous until he came along.
Whatever his name is. Damnit.
B. Kliban used this cartoon in the late 60s which is essentially the same thing.
I think the comedian was Sean Cullen. But that particular bit doesn’t seem to be on the Internet anywhere. Here’s another bit he used to do. http://www.funnyordie.com/videos/ef83fd204b/sean-cullen-food-of-your-choice-from-standupfan
I’m sure Fred Willard must have said it at some point.
What’s funny is that it was a coincidence. The theft and the murder were completely unrelated.
i recall hearing him on the radio.
Can’t remember hearing his name.
I’m almost sure his line is: "Coincidence? I don’t think so!
His routine is as you say, "…, increasingly more dramatic, ( it was woven with surprising ideas that involved, in part, unexpected math calculations.), and climbed a “ladder” of the repeated punchline.
There must be a way to find him.
From what I remember, his name was “The Todd”. The joke I remember was: “There were three Monkees on their reunion tour. There are only three Beatles still alive. Coincidence? I THINK NOT!”
Unfortunately, Googling for “The Todd” is not very productive.
“i think not” was a phrase–in use quickly becoming an obnoxious one–when I was in high school in the mid '70s. Haven’t thought of it till this thread, nor have I thought of that little brat who used to say it. Thanks, GQ.
Sadly, I can’t find any reference giving more then anecdotal evidence that the quote was linked to Newman Day before the 80’s, even though Newman Day itself certainly dates from the 70’s.
Google books shows it as a pretty common phrase going back to at least 1846. So I think we can rule out circa 1980’s stand up comics.
It was The Tad if I recall - “Lou Gehrig died of Lou Gehrig’s disease! Coincidence? I think NOT!!”
'sall I got.
ETA - ah, seeing post above - The Todd? One of those two, dammit! 
Found it in a Radford, VA. newspaper column in 1973. Just some local guy using it.
So, it certainly existed by then…
Hmmm. The catch phrase as I remember it was, “Coincidence? You be the judge!”