Her major contribution to music history was her participation in the first five seasons of the Glyndebourne Festival Opera in southern England, which was founded in 1934 by John Christie and his wife, the singer Audrey Mildmay, the conductor Fritz Busch and the director Carl Ebert. There she was a member of a renowned international ensemble and played leading roles in four Mozart operas. Three of them, the three Da Ponte's operas, were recorded and have been available ever since. They were praised by critics as outstanding and excellent. In 1934 Helletsgruber made her debut in Glyndebourne as Dorabella and Cherubino, in 1935 she also sang the First Lady in "The Magic Flute". In 1936 she sang Donna Elvira for the first time and continued to sing Cherubino and the First Lady. Even after the Annexation of Austria in March 1938 she continued to participate in the English festival.