Like many members of the kabuki community, he can trace his lineage back several generations, many members of his family being kabuki actors as well. 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, and son Ichikawa Somegorō VII are active in the kabuki theater as well, and his daughter, Takako Matsu is an experienced film actress. In addition, Kōshirō has a number of disciples, including Matsumoto Kingo III, Matsumoto Kōemon I, and Ichikawa Komazō XI.
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. He has played, among many other roles, Benkei in Kanjinchō, Kōchiyama Sōshun in Kōchiyama, Kumagai Jirō Naozane in Kumagai Jinya, and Matsuōmaru in Sugawara Denju Tenarai Kagami.
In 1970, Kōshirō appeared in the lead role in Man of La Mancha on Broadway in New York. Also, he has appeared as the King of Siam in The King and I in the West End in London. Other stage roles have included Motl in Fiddler on the Roof, the title role in Sweeney Todd, Salieri in Amadeus, Kitagawa Utamaro in the musical Utamaro, and Zeami Motokiyo in Musical Zeami.
His film credits include Emperor Hirohito in Japan's Longest Day (日本の一番長い日, Nihon no ichiban nagai hi), in which famous actor Toshiro Mifune played General Korechika Anami, Ii Naosuke in Samurai Assassin (侍, Samurai), and a number of other jidaigeki (samurai period films).