While stationed at the Dixie Mission in Yan'an, Ariyoshi met and worked with both Chinese and Japanese Communists, including Mao Zedong and Sanzo Nosaka. His primary duties in Yan'an were to learn more about the Communist efforts to train Japanese prisoners of war, translate Japanese source materials and develop Allied propaganda against the Japanese. In China, Ariyoshi saw the differences between the peasants' meager lifestyle under the Kuomintang and the Communist Party of China and left China with an appreciation for what communism or at least progressive socialism could accomplish.
Ariyoshi returned to Hawaii in 1948 and, inspired by the progressive Japanese-language paper, Hawaii Hochi, began publishing a labor-oriented newspaper, the Honolulu Record. As editor, Ariyoshi lambasted labor conditions for the working class and addressed what he considered to be other social inequalities in the islands. His socialist views bolstered the growth of the local labor movement and the Democratic Party in Hawaii. However, at the height of the Second Red Scare and McCarthyism, he and six other progressives were arrested under charges of attempting to overthrow the American government under the Smith Act. The case later became known as the Hawaii Seven. Ariyoshi spent one night in jail, and after his release, he continued to promote his socialist views through his paper. The court found him guilty, but he appealed the ruling and was eventually acquitted of all charges. Madame Sun Yat-sen, the widow of Sun Yat-sen, "the George Washington of China," had donated her mother's wedding dress to be sold to provide funds for Ariyoshi's legal defense. In 1958, Ariyoshi was forced to close his newspaper for lack of funds. He became a florist, earning himself the nickname, "The Red Florist." In 1969, he was appointed a member of the Hawaii Foundation for History and the Humanities. Ariyoshi was later appointed president of the organization, a position that he held for three years. With the American opening to China, Ariyoshi was one of the first Americans invited to return to that country, even before President Nixon's visit. He wrote a series of articles for the Honolulu Star-Bulletin, and a television documentary on Chinese arts and crafts, which helped to repopularize cloisonne in the country. He was a founder of the Hawaii-China People's Friendship Association to improve relations between China and the United States. He served on the national steering committee of the U.S.-China Friendship Association from its founding in 1974 until his death. In later life, Ariyoshi began teaching at the University of Hawaii in the Ethnic Studies Department. In 1976 the Hawaii Legislature honored Ariyoshi for his life's work and apologized for the things that had been done to him in the McCarthy era. Koji Ariyoshi died later that year, and an annual award in his name was established by the U.S.-China Peoples Friendship to honor an outstanding figure in promoting U.S.-China friendship.
In film
On May 5, 2005, he was featured in the Biography Hawaii series airing on PBS Hawaii. The documentary featured original footage of Ariyoshi during his time in Yen'an and explored his persecution during the 1950s.