Einar Aaron Swan, born Einar (Eino) William Swan on March 20, 1903, in Fitchburg, Massachusetts, was an American musician, arranger, and composer. He was the second of nine children born to Finnish immigrants. Swan's father was an amateur musician, and by his early teens, Swan was proficient in playing the violin, clarinet, saxophone, and piano. At 16, he led his own dance band, Swanie's Serenaders, traveling around Massachusetts. Although his main instrument was initially the violin, he later switched to the alto saxophone.

In 1924, bandleader Sam Lanin invited Swan to join his orchestra at New York's Roseland Ballroom, where he played with notable musicians like Red Nichols. Swan also arranged music for Fletcher Henderson's orchestra. After five months with Lanin, Swan joined Vincent Lopez's band in 1925, touring England and collaborating with musicians such as Mike Mosiello and Xavier Cugat. By 1930, he shifted focus from performing to arranging music for radio programs and bandleaders like Dave Rubinoff.

Swan's most famous composition, "When Your Lover Has Gone," was written in 1931 and featured in the film "Blonde Crazy." The song became a hit, covered by artists like Louis Armstrong and Frank Sinatra. In 1939, he co-composed "In the Middle of a Dream" with Tommy Dorsey and Al Stillman.

Swan married Ann "Billie" Kaufman in 1925, converting to Judaism and changing his middle name to Aaron, which led to a permanent estrangement from his father. The couple had two children, Pearl and Donald. Einar Aaron Swan died of a cerebral hemorrhage on August 8, 1940, in Greenwood Lake, New York, at the age of 37. After his death, Frank Sinatra donated his royalties from "When Your Lover Has Gone" to Swan's widow, Ann.