The eldest in an orphaned family, Moore worked as a longshoreman and building worker during the late 1930s and early 1940s. One of the foremost proponents of post-war rural blues, he began performing in Baton Rouge bars under the name Harmonica Slim. He later accompanied Lightnin' Slim, his brother-in-law, both live and in the studio, before commencing his own recording career in 1957. Named Slim Harpo by producer Jay Miller, the artist's solo debut coupled "I'm A King Bee" with "I Got Love If You Want It". Influenced by Jimmy Reed, he began recording for Excello and enjoyed a string of popular R&B singles which combined a drawling vocal with incisive harmonica passages. Among them were "Rainin' In My Heart" (1961), "I Love The Life I Live", "Buzzin'" (instrumental) and "Little Queen Bee" (1964). These relaxed, almost lazy, performances, which featured an understated electric backing, set the tone for Moore's subsequent work. His warm, languid voice enhanced the sexual metaphor of "I'm A King Bee", which was later recorded by the Rolling Stones. The same group also covered the pulsating "Shake Your Hips", which Harpo first issued in 1966, while the Pretty Things, the Yardbirds and Them featured versions of his songs in their early repertoires. Harpo enjoyed a notable US Top 20 pop hit in 1966 with "Baby Scratch My Back" (also a number 1 R&B hit), which revitalized his career. Never a full-time musician, Harpo had his own trucking business during the 1960s, although he was a popular figure in the late 1960s blues revival, with appearances at several renowned venues including the Electric Circus and the Fillmore East; he suffered a fatal heart attack on 31 January 1970.