Manninen was born in Kangasniemi as a farmer's son. He was one of the ten children of Topias and Matilda Manninen. After passing the
matriculation examination in 1892, he was admitted into the
University of Helsinki, receiving his Bachelor of Arts degree in 1897. From 1898-99 he was an assistant editor of the periodical
Valvoja. His early translations of Heinrich Heine were published in
Koitar, the student magazine of the university, in 1897. Parts of Heine's
Saksanmaa (
Deutschland. Ein Wintermärchen) were published in
Valvoja in 1900 while the complete translation was published in 1904. From 1907–1909 Manninen worked for the
Finnish National Theatre. In 1913 he became a teacher of Finnish language at the University of Helsinki, where he remained until 1937. Manninen was a member of editorial staff of several non-fiction books, including an encyclopedia named
Tietosanakirja (1909–21), and the chairperson of the National Council for Literature for nearly fifteen years. During the
Civil War (1918), Manninen supported the legal government and was a member of the Civil Guards. On 10 February 1907, Manninen married
Anni Swan, a noted author of fairy tales and children's books. They had three sons, Antero (1907–2000), Sulevi (1909–36), and
Mauno (1915–1969). From the 1910s on, the Manninen family spent their summers on the island of Kotavuori in
Puulavesi, where they built a house. In the serene environment of Kotavuori, Manninen spent most of his time writing poems and completing his translations. In 1939, he received the
Hungarian PEN Club medal. Manninen died in Helsinki on 6 April 1950 after a short period of illness. In 1954, his statue by the sculptor
Wäinö Aaltonen was installed in
Mikkeli. ==Literary works==