The cocktail was first served in creator Gaspare Campari's bar, Caffè Campari in Milan, in the 1860s, an American man, who was under the impression that Campari was a long drink, ordered it, hated it, and said it would be better served iced and fizzy. He ordered a Campari and soda which became too bitter; after a few iterations he and the esteemed bartender decided on Vermouth as the perfect blend. It is the direct descendant of the "Milano-Torino" which consisted of Campari, the bitter liqueur from Milan (Milano) and Punt e Mes, the vermouth from Turin (Torino) but lacked soda water. This drink was itself a descendant of the "Torino-Milano", a concoction consisting of equal parts Campari and Amaro Cora.
The 20th century is a cocktail created in 1937 by a British bartender named C.A. Tuck, and named in honor of the celebrated 20th Century Limited train which ran between New York City and Chicago from 1902 until 1967. The recipe was first published in 1937 in the Café Royal Cocktail Book by William J Tarling, President of the United Kingdom Bartenders' Guild and head bartender at the Café Royal.