[1] Masamune was born in Bizen, Okayama Prefecture, as the eldest (and sickly) son of an old and influential family of landowners.

[2][3] In 1896 he joined the English department of the Tokyo Senmon Gakko (now Waseda University), and was baptized as a Christian by priest Uemura Masahisa the following year.

[1][2] After graduation, he worked in the university's Publishing Department, and began writing literary, art, and cultural criticism for the Yomiuri Shimbun newspaper in 1903.

[1] Masamune's early writings in particular have repeatedly been described as nihilistic and bearing a negative view on life and its delusions, and his criticisms called cynical.

[4] Masamune wrote in a variety of genres; major works include the stories Ushibeya no nioi ("The Stench of the Stable") and Shisha seisha ("The Dead and the Living"), both 1916, the play Jinsei no kōfuku ("The Happiness of Human Life"), 1924, and the 1932 criticism collection Bundan jimbutsu hyōron ("Critical Essays on Literary Figures"),[1] which was expanded into Sakka ron ("A Study of Writers") in 1941–42.