Mako in "Columbo: Muder Under Glass" (1978)
According to an essay by Yumiko Murakami, author of a Mako Iwamatsu book (in Japanese) titled "East Meets West: Mako and Susie's US-Japan Stories" (1993), in the August 1, 2006 Asahi Shimbun, Mako didn't care much for his film roles. He did movies in order to support East West Players, which he co-founded in Los Angeles in 1965.
Unfortunately I'm not familiar with Mako's theater work because I've always lived in Tokyo. I am familiar, however, with some of his numerous guest starring roles in TV shows.
Some of my favorite Mako moments on TV:
1) Mr. Ozu, a Japanese businessman in the episode "Murder Under Glass" (1978) of "Columbo." The murderer is a French food critic, who entertains Mr. Ozu with fugu and geishas. At home. Where Columbo shows up and gets a clue from Ozu. But really, if you just get in L.A. from Tokyo like Ozu did, would you eat fugu there? I don't think so...
2) Master Zee, a martial arts master of "Mr. Monk vs. The Cobra" (2005) in "Monk." The story wasn't that special, but Mako was billed as "Special Guest Star," so I was happy.
3) Sam Tanaka, a publisher/agent in the episode "Author, Author" (1994) in "Frasier." Brothers Frasier and Niles try to coauthor a book about sibling rivalry -- the idea pitched by Sam, played by Mako. An unusually interesting and intellectual role for Mako -- or any Asian male actor, who usually gets to play a martial arts master or an Asian gangster or a boring/pathetic/disgusting Japanese businessman.
4) Mr. Moroshima in the episode "Sword of Honor, Blade of Death" (1982) in "Quincy, M.E." Mako, the father whose undercover cop son gets killed by the Yakuza, swears to exact revenge. The murdered cop was best friends with Sam Fujiyama, Dr. Quincy's Japanese assistant.
I didn't like the story much, but I always loved the show, mostly because of Sam, played by Japanese Canadian dancer/actor Robert Ito.
5) Chinese doctor Lin Tam, one of the four roles he played in "M*A*S*H" over a seven-year period. I don't remember the details, except that he had studied at University of Illinois.
6) Martial arts master/gang boss Low Sing in the episode "The Preying Mantis" (1966) in the cult series "The Green Hornet." A great fight between the young Mako and equally young and cute Bruce Lee's Kato!
7) Mako was Tozan, a samurai kind of guy, in the truly silly episode called "The Arrow That is Not Aimed" (1983) of Tom Selleck's "Magnum, P.I." Tozan loses an expensive Japanese plate which he was to deliver to somebody. When he finds it broken, he tries to commit seppuku. I mean, I can hear him say, "I'm only doing this to support my family and my theater..."