Born in Lowe Bavaria, Johannes Kastl was a member of the Regensburger Domspatzen and the Windsbacher Knabenchor. He received his formal musical education at the Mozarteum in Salzburg and Hamburg and taught piano and singing in various music schools as a scholarship holder of the Gustav Schickedanz Foundation. While studying towards a postgraduate qualification in singing education in Wiesbaden, he appeared in numerous musicals, operettas and opera production at the Wiesbaden State Theatre, including Bert Barry in "42nd Street", Malcom McGregor in "The Full Monty – Ganz oder gar nicht", Jeff Delpe in the world premiere of "Superhero" and Lewis in "Our House". In addition, he performed the lead role in "Zwei hoffnungslos verdorbene Schurken" (Lawrence Jameson) and also appeared in Zimmermann's opera "Die Soldaten".
During the 2017/18 season, he worked as Assistant Director on the productions "Jesus Christ Superstar", "Tannhäuser", "Die Zauberflöte", "Jephtha", "Un ballo in Maschera" and "Liliom".