Appl was born in Regensburg, Bavaria, then in West Germany, in 1982. He began his singing career in the Regensburger Domspatzen, a local boys' cathedral choir with a national and even international reputation. Appl went on to the Munich University of Music and Theater, working with Helmut Deutsch and also earning a business degree in Munich at the same time. Appl rounded out his vocal training at the Guildhall School of Music and Drama in London, studyig with Edith Wiens and Rudolf Piernay, and working privately with Fischer-Dieskau almost until the baritone's death in 2012. Appl has performed in operas from the mid-Baroque (Purcell's Dido and Aeneas) to contemporary times (Péter Eötvös' Tri Sestri). He enthusiastically performs the standard lieder repertory, but he has also sung vocal works written for him by an unusually large variety of composers, including György Kurtág, Nico Muhly, and Susan Oswell. Appl has appeared with leading international orchestras, including the NHK Symphony Orchestra, NDR Radiophilharmonie, and Philadelphia Orchestra, and he is also an established performer with historically oriented ensembles, such as the Akademie für alte Musik, Concerto Köln, and the Stuttgart Bach Collegium. In 2012, he sang for Pope Benedict XVI at the latter's summer residence, Castel Gandolfo.
After a song recital released on the Champs Hill label in 2016, Appl signed that year with the Sony Classical label and has issued several critically acclaimed albums. These include The Secret Fauré III in 2020. He was also heard on an installment of Hyperion's complete cycle of Brahms songs, Vol. 7 with accompanist Graham Johnson. Appl has received several major awards, including the Schubert Prize in 2012 and the Gramophone Award as New Artist of the Year 2016. ~ James Manheim, Rovi