Hatoyama was chief cabinet secretary to Prime Minister Tanaka Giichi's government from 1927 to 1929. From 1931 to 1934, Hatoyama served two successive governments as education minister and in the meantime became a leading member of the conservative Seiyukai Party. As proof of his conservative mettle, when a prominent Kyoto Imperial University professor was attacked for his liberal views by right-wing ideologues in 1932, Hatoyama forced him to resign his university position the next year. Hatoyama was also a member of the Taihei Yokusan-kai (Imperial Rule Assistance Association, IRAA) during 1942–1943.
After World War II, Hatoyama organized the conservative postwar Liberal Party and became its first president. Because of his past right-wing politics, however, he was banned from public life by order of the supreme commander for the Allied powers, General Douglas MacArthur, who controlled the postwar Japanese occupation. Hatoyama was forced to leave his newly formed party in the hands of Yoshida Shigeru. After the occupation, Hatoyama returned to politics in late 1951 and founded the Japan Democratic Party (JDP), becoming its president in 1954. After engineering his own political rehabilitation and with the aid of the JDP, Hatoyama ousted Prime Minister Shigeru's government and became prime minister in 1954.
At the time, a group of leading Liberal Democratic Party (LDP) politicians sought to revise what they considered to be the coercively imposed MacArthur Constitution. Hatoyama, no fan of MacArthur or of the U.S. occupation, became one of the leading proponents of a constitutional revision. Under Hatoyama's leadership, Japan once again established itself within the international community and in 1954 began making reparation payments to nations it had attacked or occupied prior to its 1945 surrender. In 1956 the Hatoyama government negotiated a termination of hostilities agreement with the Soviet Union, which then dropped its United Nations veto against Japan's membership in the organization.
In failing health, Hatoyama resigned from office in 1956 and was succeeded by Ishibashi Tanzan. Hatoyama died on 7 March 1959 in Bunkyouku, Tokyo.
Hatoyama Ichirō. Hatoyama Ichirou Kaikoroku [Autobiography of Hatoyama Ichirou]. Tokyo: Bungeishunjusha, 1957.; Schaller, Michael. Altered States: The United States and Japan since the Occupation. Oxford: Oxford University Press, 1997.