The drink has been documented in Italy since the late 1940s, and became popular in the 1950s, but the origin is uncertain, and early recipes differ somewhat from the modern standard.

The IBA recipe for the negroni specifies that it be built over ice in an old-fashioned or rocks glass and garnished with a slice of orange, similar to an old fashioned or spritz (short, minus the soda).

The drink's origins are not known with certainty, and one must distinguish the modern recipe (an equal-parts cocktail of gin, vermouth rosso, and Campari, served over ice) from the name "negroni".

In the late 1940s the short drink then acquired the name negroni from a separate, similar long Italian-style drink of vermouth and soda, with small amounts of Campari and gin, served over ice; or from a variant of the Milano–Torino or Americano, equal parts vermouth and Campari, with a small amount of gin, plus soda, served over ice.

This drink is listed in numerous American, French, and Spanish cocktail books of the 1930s and 1940s, including Boothby (1934, p. 39) (shaken, twist lemon peel over), Brucart (1943), and Trader Vic's Bartender's Guide (1947).

It still differs from the modern negroni in being stirred, not built; implicitly served up, not on the rocks; and garnished with a lemon twist, not an orange slice.

The earliest reports in English are from traveler writers to Italy and the Mediterranean, and describe a long drink based on vermouth and soda, with the addition of small amounts of Campari, gin, and sometimes Angostura bitters, similar to a vermouth-based spritz.

The commonly-held origin story is that it was concocted by a member of the Negroni family asking the bartender to strengthen the Americano by adding gin, rather than the normal soda water.