Most origin stories of cocktails tend to be subject to debate and the Negroni is no exception. This iconic cocktail is a favourite in any good bar around the world, so it's easy to see why multiple parties claim it as their own creation.
There’s no actual historical account (Italians never really wrote anything down) but it is generally believed that Count Camillo Negroni invented the drink when he ordered an Americano made with gin in place of the usual soda water (what a booze hound).
The bar in which he ordered the iconic drinks serve is Caffe Casoni, in Florence, where the Count had returned to during the prohibition era in the United States. (#mood.)
There are multiple other stories and theories as to the origin of the Negroni cocktail. And you can find many more stories about the cocktail origins on Difford's Guide.
It soon caught on and is now the go to cocktail for bartender knock offs everywhere in the world.