As to its origin, the most prevalent story is that it was first created in Florence, Italy in 1919, at Caffè Casoni, an antique shop/bar, (think Il Buco) where now Caffè Giacosa is found.
As the story goes Count Camillo Negroni asked bartender Fosco Scarselli to strengthen his favorite cocktail, the Americano, by adding gin instead of the typical soda water. Fosco distinguished the drink with an orange garnish rather than the typical lemon.
A Brief History of the Negroni