Manninen translated the works of Homer, Sophocles, Euripides, Heine, Ibsen, Petőfi and Runeberg into Finnish.

After passing the matriculation examination in 1892, he was admitted into the University of Helsinki, receiving his Bachelor of Arts degree in 1897.

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.

On 10 February 1907, Manninen married Anni Swan, a noted author of fairy tales and children's books.

It included several poems, such as Nyt (now) and Kaukainen tie (distant road), which dealt with the loss of his son Sulevi.

He translated a number of works of the Swedish-speaking national poet of Finland, Johan Ludvig Runeberg.

He along with Juhani Aho, Paavo Cajander, Arvi Jännes, Eino Leino and Alpo Noponen translated Runeberg's Fänrik Ståls sägner (Vänrikki Stoolin tarinat in Finnish) in 1909.