In 1962, he set up the Harold K.L. Castle Foundation, which remains the largest private foundation based in Hawaiʻi.[2] He and his foundation have donated large amounts of land to educational and other public institutions, including Hawaii Loa College, ʻIolani School, Castle High School, Kainalu Elementary School, Castle Medical Center, and the Mokapu peninsula land that became Marine Corps Base Hawaii.[3]
He and his wife, Alice Hedemann, both graduated from Punahou School; he served on its Board of Trustees (1922–1937), and their descendants have continued to attend and support the school.[2]