Mikuni was born Masao Sato in Ōta, Gunma Prefecture, and grew up in Nishiizu, Shizuoka Prefecture, the hometown of his adoptive father.[3] After dropping out of Toyo Junior High School in Shimoda, he led a wandering life between Japan, China and Korea (then under Japanese control).[3]
At the age of twenty, Mikuni received call-up papers for the Japanese military.[3] In his autobiography and in interviews Mikuni claimed that he tried to escape the draft as he didn't want to die or kill people, but was caught after his mother had informed the police for fear of ostracism.[5][6] He was sent to the Chinese front, fell ill, served his time in a unit of unfit and incompetent soldiers, and never fired a weapon at the enemy.[5][6]
After returning to Japan, he worked in a variety of jobs.[3] His career as an actor started when he participated in a newcomer contest by the Shochiku film studio in 1950, although he originally had no intention of becoming an actor.[1][5] He gave his debut in Keisuke Kinoshita's The Good Fairy (1951), for which he received the Blue Ribbon Award for best newcomer, and took his stage name from the character he played in the film.[3][7]