Leonhardt became involved in politics in 1932 when he joined the National Front, and before long he had risen to the rank of Gauführer (equivalent to Gauleiter) in both Basel-City and the Canton of Solothurn.
The Staffelführer in turn answered to a local SS-Gauführer, or district leader, who answered to the national leader of the SS, known as the Reichsführer-SS.