It was created sometime around 1920 by bartender Fosco Scarselli at Bar Casoni in Florence. It was first made for, and named after, Count Camillo Negroni who frequented Bar Carsoni and drank Americanos. The story goes that he requested a stronger, more potent and less watery Americano. Fosco obliged by adding gin and removing the soda.
On January 17 1920 prohibition came into effect in the US as a result of the 18th amendment.