Born James Moore in Lobdell, Louisiana, 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 Records, 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). On these hits he was accompanied by understated electric backing from the regular stable of Excello musicians • including Lazy Lester, whom Harpo influenced.
The singer was known as one of the masters of the blues harmonica; the name Slim Harpo was a humorous takeoff on slim harp, the popular nickname for the harmonica in blues circles.
Harpo was the point man of the 1950s Louisiana Swamp/Blues movement. Harpo, along with Lightning Slim, Lazy Lester, Lonesome Sundown, and a dozen other downhome artists, recorded for A&R man J.D. Miller in Crowley, Louisiana. The records were then issued on the Excello label, based in Nashville.