[3] His short stature, soft features, and expressive face and voice suited him for rebellious “younger brother” roles, and he appeared as a youth lead in many silent and early sound films, notably in several Yasujirō Ozu classics[4] and the “Yota” series, about the antics of a trio of young idlers that also included Akio Isono and Shōzaburō Abe.

Mitsui was reportedly chastened by the experience and resolved not to drink anymore while performing; he won the award the following year and began the most fruitful and highest-profile period of his career.

[18] Apart from his informal wartime performances, Mitsui did not appear onstage professionally until 1964,[17] when he was asked to co-star in two prestige productions at the Toho Geijutsuza (Art Theater), which was located in the studio's Tokyo office building.

In poor health throughout the 1970's, he primarily appeared on television programs, notably starring as the family patriarch in the 1969-70 season of the popular Fuji TV drama Oyome-san (Bride).

[21] Dispirited by the diminishing effect that television had on film production at Shochiku, which at one time had been nicknamed "House of Mitsui" due to his commitment and influence (and as a joking reference to the famous Japanese family to which he bore no relation),[17] he sold his longtime home near the Ōfuna studio and moved his second wife, former Takarazuka Revue actress Shinobu Omori, and the daughter they had adopted to another part of Kamakura.

[3] With these two awards for The Lower Depths, Mitsui was able to distinguish himself among the top performers in Japanese cinema, whom Kurosawa had selected and dress-rehearsed on-set for 60 days to create the ultimate acting ensemble.

In addition to his many performances for prominent directors such as Kurosawa, Ozu, Kobayashi, and Kinoshita, Mitsui is best known to Western audiences as the duplicitous village elder in Hiroshi Teshigahara's Oscar-nominated[32] Woman in the Dunes (1964), for which he received above-the-title billing on the original film poster along with stars Eiji Okada and Kyōko Kishida.

[33] Mitsui's portrayal of the lazy nihilist in The Lower Depths is well-remembered as a showcase for his improvisational talents and his “oboe-like,”[13] “beautiful voice with its unique charm and sense of rhythm.”[35] The film's final act becomes a tour de force for Mitsui, who mockingly impersonates Bokuzen Hidari (whose character's humanistic influence has been defeated by the gambler's cynicism), leads the remaining denizens in song, and ends the film with his brutal remark.

He is real and relaxed, with no sense of pride or regret.”[36] Mitsui's largest part for Kurosawa after The Lower Depths was the lead journalist who comments on the wedding reception that opens The Bad Sleep Well (1960); his role as a sarcastic observer was noted by Kurosawa scholar Donald Richie to parallel traits of Yoshisaburo the gambler in the prior film,[37] and Mitsui's “particularly enthralling”[38] performance helped to associate his legacy with sardonic characters as well as boozy ones.

[41] A 1995 issue of the Bungeishunjū monthly No Side commemorating Japan's great postwar actors included a full-page essay by the writer Midori Nakano celebrating the familiar clear-eyed urban attitude of the characters Mitsui played for Kurosawa, particularly his role in The Lower Depths.

"[43] In 2000, the rakugo star and film critic Shiraku Tatekawa named Mitsui one the top three Japanese actors of the 20th century, calling him "addictive," "haunting," and "unforgettable.

"[49] In his February 19, 2024 "Cigarette Burn“ column posted in the webzine Kemur, author Daisuke Kodama called Kōji Mitsui the quintessential "Shōwa smoker" due to his deftness with match-lighting and blowing of smoke rings in the film Floating Weeds.