Color me surprised. I’d always thought - or heard somewhere - that it was the same thing as Russian dressing, but the name had been changed during the “red scare” of the 1950’s, when the stuff first seemed to become popular (I’ve always associated thousand island dressing with the 1950’s-1960’s era diners where it always came on your salad whether you’d ordered it or not). I assumed that “thousand island” was just a catchy name dreamed up by an ad agency somewhere. Sort of like “freedom fries”.
SS
Southeastern NY for me and I had a vague idea it was something to do with the St. Lawrence.
I first heard about this many years ago in a short story, “The Thousand Islands Snatch,” by Nicholas Monsarrat in his collection “The Ship That Died of Shame.”
I learnt about the dressing later (I don’t live in the US ).
Somewhere in this house is a 8mm camera with a roll of undeveloped film of my family’s trip to the Thousand Islands circa 1981.
Well, I’ve never been big on salads or salad dressing. But I think I made the connection the time I went to Kingston for a short vacation by myself, and took a boat cruise that went around the Thousand Islands, or some of them anyway.
(That was the morning before the 2003 blackout, as it happens.)
I was up in Kingston, Canada visiting my brother in the early 1990s and we toured some ruins, which had a graphic about Thousand Island dressing. So, since about then and there.
Me neither.
I’m pretty sure I knew what the Thousand Islands were before I ever heard of Thousand Island dressing. I grew up in western New York, and my family took a camping vacation to the area when I was 5 or 6. I was absolutely fascinated with Boldt Castle. I wanted to buy it when I grew up and finish building it. Little did I know that it would take $15 million just to restore it to the way it was in 1904 when construction was halted.
Always knew.
Age six or seven. I was: 1. already a map nerd, and 2. living in New York state (albeit far from the St. Lawrence Seaway).
?? Are you confusing it with the Land of a Thousand Lakes? Unless by ‘environs’ you mean, say, North America. 
Actually, Minnesota is the “land of 10,000 lakes.” I know this because an ex-girlfriend lived there, and her parking lot frequently flooded so badly that she called it “the ten-thousand-and-first lake.”
Lakes, islands, tomato, tamahto - same same.
The “land of a thousand lakes” is Finland. (Although it’s an understatement. By the latest count there are actually 187,888 of them, if a lake is defined as larger than 500 square metres.)
Since I was a young child, because I partially grew up in Montreal and spent time in the Ontario/upstate New York area.
In fact it’s possible I knew of the Thousand Islands recreation area before I heard of the dressing. I don’t remember.
This 
My grandparents had a summer house in Canada and the shortest way to get there was driving along the lake and scooting up and across the border at the thousand islands. Also, he liked fishing in the thousand islands so we frequently took the Sea Gypsy there for 3 or 4 weeks every summer. Lovely, fantastic, seriously good memories.
The area of Wisconsin in which I grew up has its own “Thousand Islands” area, a bend in the Fox River near Kaukauna. There was (and still is) an “environmental center” in the area that I visited on field trips for school and Cub/Boy Scouts. But I don’t think I ever linked the dressing with the name of the local area in my mind, never mind the name-sake area in New York/Canada.
I liked Thousand Island dressing as a kid, but I don’t have the first clue if I ever asked my Mom or Dad where the name came from.
Another Upstater who’s always known. Russian dressing is better than Thousand Island, but a few years (or two decades) ago it disappeared from store shelves.
The Thousand Islands in Ontario? My grandparents lived in Belleville, Ontario, and while going there every year, we’d pass signs for the city of Brockville, “City of the Thousand Islands.” So that’s always how I’ve thought of the Thousand Islands, ever since I can remember.
Until two minutes ago, I thought the same. That it related the the myriad of tiny islands in the south Pacific.
Now I know better, I guess.