The fourth son of Tetsuo Onchi, Koshiro Onchi was born into Tokyo aristocracy. His father tutored the three princes chosen to wed emperor Meiji’s daughters. After failing his high school entrance exam in 1909, Onchi pursued oil painting at the Hakubakai school. He enrolled in Tokyo School of Fine Arts in 1910, first pursuing oil painting, followed by sculpture. In October of 1913, Onchi and classmates Kyokichi Tanaka and Shizuo Fujimori seeded Tsukubae, planning the print and poetry magazine that would become an early medium for the Sosaku Hanga movement. After graduating in 1914, he continued magazine work.
Onchi published his first series of prints, Happiness in 1917, participating in the 1919 inaugural Nihon Sosaku Hanga Kyokai exhibition. In 1921, he began art-focused Naizai, along with Kenji Otsuki and Fujimori. Onchi dedicated himself to the promotion of printmaking, namely “creative prints,” as a legitimate art form. In 1928, following Lindbergh’s transatlantic flight, a newspaper hired Onchi to go up in a plane and record the experience in Sensations of Flight (1934). Though he made his living through illustration, he contributed to many magazines. He created book covers over the years and published multiple books of his own poetry. In 1949, Onchi received Japan’s first prize for book design. A leader and mentor of the Sosaku Hanga movement, he headed Ichimokukai, a monthly meeting of woodblock artists, from 1939 until the end of the occupation. He belonged to many progressive art movements including the League of Japanese Artists, Japan Abstract Art Club, and the International Print Association. Onchi’s early works may be signed “onzi.”