Who Invented the Old Fashioned
Nobody invented it, because it was the original drink the word "cocktail" named. The name came later, when people started asking for the simple build the old-fashioned way.
A man walks into a bar around 1880 and orders a cocktail. He does not get spirit, sugar, water, and bitters. He gets something with a splash of liqueur, a curl of citrus, maybe a dash of absinthe, whatever the bartender has been showing off with that month. So he tries again, more specific this time: make it the old-fashioned way. Just the whiskey, the sugar, the bitters. That request, repeated often enough, became a name.
That is the honest short version of where the Old Fashioned comes from. It was not invented. It was rescued, and the rescue gave it a name.
The drink is older than its name
The Old Fashioned is, by build, the original cocktail. We can say that with some confidence because of where the word "cocktail" first shows up in print. In 1806, a newspaper in Hudson, New York answered a reader who wanted to know what a "cock-tail" was. The editor's reply: a stimulating liquor composed of spirits of any kind, sugar, water, and bitters.
Spirit, sugar, water, bitters. Read it twice. That is the Old Fashioned, defined eighty-some years before anyone called it that. For the first stretch of the nineteenth century, "cocktail" meant exactly that template and nothing else. A drink with citrus was a sour. A long one over ice was a julep or a sling. The cocktail was the austere one.
So the Old Fashioned has no inventor for the same reason a handshake has no inventor. It was the baseline. Everything fancier was a departure from it.
Where the name actually comes from
By the late 1800s, bartenders had a wider shelf and an instinct to use it. Curaçao, vermouth, maraschino, absinthe, and a dozen syrups crept into drinks that still went by the name cocktail. For a drinker who wanted the plain old build, the easiest way to ask was to ask for it the old-fashioned way. The adjective stuck to the drink and turned into a proper noun.
This part is well supported by the printed record. The structure of the drink, the simple build, predates the name, and the name reads like exactly what it is: a request for the original, made at a moment when the original had started to feel quaint.
The story you have probably heard
Here is where you have to be careful. The most repeated origin tale credits the Pendennis Club, a gentlemen's club in Louisville, with inventing the drink in honor of a local bourbon distiller, who then supposedly carried it to a famous bar in New York. It is a tidy story. Bartenders love telling it. It puts a name and a place on something that otherwise has neither.
The trouble is that the documentation is thin. The tale appears well after the fact, the dates do not line up cleanly with when the drink and its name were already in circulation, and no contemporary record nails it down. Treat it as bar lore, not history.
A good origin story and a true origin story are different products. The Pendennis tale is the former, repeated until it sounds like the latter.
None of this should disappoint you. The real answer is better. The Old Fashioned is what you get when you ask for the first drink the word "cocktail" ever described, made plainly, the way it was made before the shelf got crowded. There is no inventor to thank. There is just two hundred years of people, every so often, asking for the simple thing.
So when you stir whiskey with a little sugar and a couple dashes of bitters, you are not making someone's signature creation. You are making the baseline that everything else wandered away from, and then came back to.