He grew up in Kona, and in seventh grade moved back to Honolulu, where he attended McKinley High School and graduated at the top of his class in 1924.

[1] After graduating from high school he earned a bachelor's degree in economics with a minor in philosophy from the University of Chicago.

[3] After Pearl Harbor was attacked in 1942, Marumoto helped to start the Emergency Services Committee,[4] which worked with law enforcement and civilians to ease tensions between the Japanese-American community and those who thought they were a threat.

Instead, he taught at the Military Intelligence Service (MIS) language school since he was fluent in Japanese.

[7] He became president of the Hawaii Bar Association in 1954 and was the first Japanese American to serve in that position.