A traditionally made Negroni is stirred, not shaken, and built over ice in an old-fashioned or rocks glass and garnished with a slice of orange. Outside of Italy an orange peel is often used in place of an orange slice.
While the drink's origins are unknown, the most widely reported account is that it was first mixed in Florence, Italy, in 1919, at Caffè Casoni (formerly Caffè Giacosa), on Via de' Tornabuoni and now called Caffè Roberto Cavalli. Count Camillo Negroni concocted it by asking the bartender, Fosco Scarselli, to strengthen his favorite cocktail, the Americano, by adding gin rather than the normal soda water. The bartender also added an orange garnish rather than the typical lemon garnish of the Americano to signify that it was a different drink.
After the success of the cocktail, the Negroni family founded Negroni Distillerie in Treviso, Italy, and produced a ready-made version of the drink, sold as Antico Negroni 1919. One of the earliest reports of the drink came from Orson Welles in correspondence with the Coshocton Tribune while working in Rome on Cagliostro in 1947, where he described a new drink called the Negroni, "The bitters are excellent for your liver, the gin is bad for you. They balance each other."
Cocktail historian David Wondrich researched Camillo Negroni, whose status as a count is questionable, but whose grandfather, Luigi Negroni, was indeed a count.