The most historically factual but still slightly doubtful origins of this cocktail is that the drink is named after that of Cammillo Luigi Manfredo Maria Negroni. When he went to Café Casoni and asked the bartender Fosco Scarselli to add gin to his Americano (Campari, Itlaian vermouth and soda). This would happened around 1919 because a letter was sent in October 1920 to Negroni talking about his own drink. The drink tends to be a ratio of 1:1:1 of Gin, Campari and sweet vermouth but it is a drink, like the martini which you can style your way, less bitter? Less Campari. Less sweet? Less vermouth.
Count Camillo Negroni during his twenties he travelled round America, even ending up as a cowboy at one point! He returned to Italy during the Prohibition era. After the success of the Negroni his family opened up a distillery (1919) which produced the drink ready made.
Although in some shock twist the Negroni family have said and proved that in their family they have never had a Camillo in fact it was a made up name and the real Count Negroni, Pascal Olivier first created the drink in West Africa. Although like all of these cocktails their history is very murky.
The Curious Bartender: The Artistry and Alchemy of Creating the Perfect Cocktail, Tristan Stephenson