Mishima formed the nationwide Boy Scouts of Japan with Count Futara Yoshinori in 1922, with Shimpei Gotō at its helm.

In 1950, he transferred the Boy Scouts Association of Japan headquarters to his villa and home in West Nasuno, Tochigi Prefecture.

J. S. Wilson presented the Wood Badge to Mishima, which he had earned at Gilwell Park after the Austrian World Scout Jamboree.

From February 25 to March 7, 1965, he wrote his serial Scout Jūwa (スカウト十話) for the Mainichi Shimbun, his final work.

Mishima's handwriting can be found on the stone monument at 'Japan's First Boy Scout Camping Ground' on the banks of Lake Biwa in Omatsuzaki (Otsu City, Shiga Prefecture).