In Denver, Yasui married and became a local leader in civic affairs, including leadership positions in the Japanese American Citizens League.

In 2015, Senator Mazie K. Hirono nominated Yasui for the Presidential Medal of Freedom, the highest civilian award of the United States.

[1] On November 16, 2015, President Barack Obama announced that Yasui would receive a posthumous Presidential Medal of Freedom.

[4] He also was a member of the United States Army's Reserve Officer Training Corps (ROTC) program at the university, earning a commission after graduation in 1937.

[4] Following the attack on Pearl Harbor, which drew the United States into World War II against Japan and Germany, Yasui resigned his position with the consulate on December 8, 1941.

[9] After returning to Hood River, he tried to report for duty with the United States Army at the Vancouver Barracks in neighboring Washington, but wasn't allowed to take up his commission on nine occasions.

[6] Masuo Yasui was arrested on December 13, 1941, by the Federal Bureau of Investigation as an enemy alien, and had his assets frozen.

[4] President Franklin D. Roosevelt signed Executive Order 9066 on February 19, 1942, that allowed the military to set up exclusion zones, curfews, and ultimately the internment of Japanese Americans during the war.

[9][11][12] He first asked an officer on the street to arrest him but was told to "Run along home, sonny boy," and so he walked into the police station.

After the notice was given for the Japanese to evacuate, Yasui notified the authorities that he had no intentions of complying, and went to his family's home in Hood River.

[12] In 1944, Yasui was allowed to leave the internment camp in the summer and was employed in Chicago before moving to Denver, Colorado, in September 1944.

[4] In 1954, he was chairman of the Japanese American Citizens League’s (JACL) district covering Colorado, Wyoming, Texas, Nebraska, New Mexico, and Montana.