Alabama.
Arizona.
California.
Colorado.
Etcetera.
There are several states with four-syllable names, which means that any one of them, theoretically, could’ve become the standard word used (in the US) to estimate seconds when timing something.
How did Mississippi get the nod? Why do we count “one Mississippi, two Mississippi” instead of “one Indiana, two Indiana” or “one South Dakota, two South Dakota”?
Some of us say one one thousand two one thousand…
I’m guessing this is a manifestation of kids-lore. Mississippi is somewhat difficult for smallish children to pronounce (and spell!) so it is a suitable delay mechanism when eyes are closed in the beginning of hide and seek and other games.
My guess: It’s hard to speed up “Mississippi”. You might accelerate while counting “one Alabama, two 'labama, three 'l’bama” and so on, but spitting out all those s’s and p’s in “Mississippi” is pretty much guaranteed to take you a second.
you can speed up Mississippi, you just say “miss ippi” and then eventually “sippi”. Kids do that all the time when counting for hide and seek (or at least we did).
But it’s pretty objectively clear whether one says ‘sss’ or ‘ssiss’; either the ‘s’ is interrupted or it isn’t; and that can only be speeded up so much. As opposed to the question of whether there’s a long enough schwa between ‘l’ and ‘b’ in "al’bama’, which isn’t so black and white.
(Plus, if you’re not putting numbers in between, with “Alabama Alabama” the first and last sounds run together, whereas the beginning and end of ‘missisisspi’ are distinct. )
Wikipedia has a listing of words used for slowing down the count. Mississippi is in there, of course, but so are others:
https://en.wiktionary.org/wiki/Appendix:Words_used_as_placeholders_to_count_seconds
(I was taught the “one thousand” rule myself.)
The 1996 date they give isn’t the earliest. I’ve found it listed on Google N-gram in 1964 ( Telemachus Clay: A Collage for Voices - Lewis John Carlino - Google Books ), but I know from personal experience that it’s older than that. It might be hard to find recorded, though.
Just for kicks, here’s an example of using “one thousand” as a timing mechanism, from 1969. Like the 1964 use of “Mississippi”, it’s from a play:
I remember doing it as a kid - and of course I just copied it from other kids, but I remember thinking it sounded cool. There is an alliterative feel to it that isn’t present in other states.
They do similar things in other languages, but the most common seems to be to say “one thousand” in that language between numbers, or to count 1001,. 1002, 1003, etc.:
This doesn’t say how old the practice is, though.
Here’s a source putting the use back to the 1950s:
Just to be clear, I know “Mississippi” isn’t the only (or even necessarily the most popular) word people use to time out one second. But it’s definitely the most popular state (or river?) name used in US versions of this practice.
I did dig up a 1959 citation from Popular Photography magazine via Google Books, but phrased “Mississippi one, Mississippi two” —which makes more logical sense. The number of elapsed seconds is the second part of the phrase, not the first.
I’m sure the practice predates this, but print citations start turning up in the '60s.
Right. Unlike Arizona, California, or Minnesota, Mississippi is fun to say. It’s also fun to read and fun to spell.
I know, but I give you what I find. And the other “spacers” are relevant.
I don’t know why “Mississippi” got the nod as the most common state, but my naïve guess would be that it was simply because it’s a long single-word state name. But Pennsylvania’s longer and has as many syllables. I think they ought to sue.
Semi-related:
NPR’s How To Do Everything podcast recently featured a lighthearted look at the various ways people count out the seconds. The interesting bit I learned is that people varied wildly at how long they took to count off 10 seconds. As much as 40% off.
Oh, c’mon! Mississippi is the poorest state in the union! Can’t you rich guys think of anything else besides how to take things away for poor people?
Yeah, just let 'em have that one. 
Mississippi is a fun word with 'S’s and 'P’s in it, fun to say, slightly tongue twisting, and fun and easy to spell too for a long word. That’s why it’s used frequently as a word to count time, it’s known because saying it and spelling it is part of the common culture learned at a young age.
It certainly dates back to the 50s as my friends and I were doing it then. The “counter” words were Mississippi, one-thousand (you had to say both one and thousand after each number counting two one-thousand, three one-thousand, … making no logical sense) and locomotive.
In my experience Mississippi was the longest and one-thousand was the shortest. I never discussed with others whether they also thought this to be true to maintain my 'competitive edge"
And speaking of spelling Mississippi, did anyone else spell it M, I, crooked letter, crooked letter, I, crooked letter, crooked letter, I, humpback, humpback I.
Back when I was a moderator, before I would post a response to a particularly awful rule-breaker I would count to ten:
“One banhammer, two banhammer, three banhammer…” 
Great OP, BTW.
On the “Words to Time Calculator” I submitted the following script I just wrote:
Leo Bloom *(Clears throat, places left palm over chest, raises right hand as if wafting gentle breezes)*One Mississippi
Two Mississippi
Three Mississippi
I am told–https://www.edgestudio.com/production/words-to-time-calculator--that “a professional” reading this script at an average speed would take two seconds. Well spent, no doubt, but troublesome in relation to this thread’s premise.