In his effort to pursue his musical inclination, he met Max Reger, with whom he took private lessons from 1904 in Munich.
In 1911, having had his first success as a composer and having won an Arthur Nikisch scholarship, he became teacher of composition at the Stuttgart Conservatory, where he was named professor in 1916.
In 1921, together with Paul Hindemith and Heinrich Burkard, he established the Donaueschinger Kammermusikaufführungen zur Förderung zeitgenössischer Tonkunst.
After the Second World War, he became president of the Munich Hochschule für Musik und Theater, a position which he held until he became Emeritus Professor in 1950 and led the school's reconstruction after 1945.
At first, he was strongly influenced by his mentor Max Reger, whose language of polyphony and harmony also featured in Haas's music.