Born in Japan, Makoto Iwamatsu was living there with his grandparents while his parents studied art in the United States, when Japan and the U.S. went to war in 1941. His parents remained in the U.S., working for the Office of War Information, and, at the cessation of the conflict, were granted U.S. residency by Congress. "Mako", as he became known, joined his parents in New York and studied architecture.
He entered the U.S. Army in the early 1950s and acted in shows for military personnel, discovering a talent and love for the theatre. He abandoned his plans to become an architect and instead enrolled at the famed Pasadena Community Playhouse. Following his studies there, he appeared in many stage productions and on television. In 1966, he won an Academy Award nomination as Best Supporting Actor for his first film role, as the coolie "Po-Han" in The Sand Pebbles (1966). He worked steadily in feature films since.
He appeared on Broadway in the leading role in Stephen Sondheim's "Pacific Overtures", and co-founded and served as artistic director for the highly-acclaimed East-West Players theatre company in Los Angeles.
Following a long battle with cancer, Mako passed away on July 21, 2006, at the age of 72. He was survived by his wife, Shizuko Hoshi (who co-starred in episodes of M*A*S*H (1972)) as well, and his children and grandchildren.