Knudåge Riisager was born in Estonia, but his family moved to Denmark in 1900 when he was three years old. In Denmark he studied economics at the University of Copenhagen, whilst being the pupil of Otto Malling and Peder Gram, and later Albert Roussel in Paris.
From 1939 to 1950 he was Assistant Secretary of the Ministry of Finances. From 1922 to 1924 he was President of the Danish section of ISCM. In 1937 he became the President of Danish Composers' Society, member of the boards of KODA and the Society for the Publication of Danish Music. From 1942 he was a musical advisor to the Danish Radio, and from 1956 director of the Royal Academy of Music. In addition to that he was a member of Musikaliska Akademien in Stockholm.
He is the author of numerous articles in Danish and foreign periodicals. His works include opera, ballets, stage music, orchestral music (including 4 Symphonies and 9 overtures), chamber music, piano compositions, choral works, and songs.