His father and grandfather were the eighth and seventh, respectively, to hold the name of Matsumoto Kōshirō, and he traces his lineage back to his great-great-grandfather Nakamura Karoku I, if not further.

Kōshirō's brother, Nakamura Kichiemon II, son Matsumoto Koshirō X and grandson Ichikawa Somegorō VIII are active in the kabuki theater as well, and his daughter, Takako Matsu is an experienced film actress.

He made his stage debut in 1945, at the age of three, under the name Matsumoto Kintarō II, and took the name Ichikawa Somegorō VI four years later.

A graduate of Waseda University, and a versatile actor, Kōshirō IX has performed extensively not only in kabuki, but in Western stage plays, film, and television.

In 1970, Kōshirō appeared in the lead role in Man of La Mancha on Broadway in New York.