Slezak worked initially as a gardener, fitter and in the army for he with singing lessons began. He studied with a. Robinson in Brno and made his debut in 1896 in the role of Lohengrin (Richard Wagner). He worked at the opera houses of Berlin (1898-1899), Breslau (1900-1901) and Vienna (1901-19340.In addition, he performed all over the world, particularly in Wagner's opera's. So he sang for example at the Metropolitan Opera in New York City under the direction of Arturo Toscanini. He also worked with Gustav Mahler and Cosima Wagner. From 1932 he worked as an actor in German comedies.
Slezak has published several books, including his autobiography titled Mein Lebensmaerchen (1948).