 On December 15, 1957, the Cardinal was made Secretary of the Sacred Consistorial Congregation in the Roman Curia.Mimmi later chose to be advanced to the rank of Cardinal Bishop, receiving the suburbicarian see of Sabina e Poggio Mirteto on June 9, 1958.He was also one of the cardinal electors who participated in the 1958 papal conclave that selected Pope John XXIII.