The cocktail was invented in Florence, Italy in 1919 at Caffè Casoni (which still exists today as Caffè Giacosa). Count Camillo Negroni asked the bartender to strengthen the Americano (his favorite drink) by adding gin instead of soda water.
The bartender also added an orange garnish rather than the lemon of the Americano to signify that it was a different drink.
One part gin
One part sweet vermouth
One part Campari
Stir in an old-fashioned glass over ice, add a slice of orange peel to garnish.