Yoshino converted to Christianity while still in secondary school, and he soon became prominent in the Christian Socialist movement in his country. After studying abroad from 1910 to 1913, he returned home to become a professor at Tokyo Imperial University and one of the most forceful advocates of parliamentary government in the country.

Without questioning the sovereignty of the emperor, an unheard-of act at this time, Yoshino nevertheless called for a “government for the people” (minponshugi), insisting that the people's demands be the basic goal of government. To this end he advocated universal suffrage, civilian control over the army, the transformation of the House of Peers to a popularly elected body, and the gradual establishment of a socialist state.

In hopes of furthering these goals, Yoshino briefly entered politics, forming his own party, the Reimeikai, in 1918. In 1924 he resigned his university post to write for the daily Asahi shimbun, and, even after he severed that connection, he continued to write for the wider public about current affairs and problems. Yoshino also played an important part in the preservation and publication of historical sources for the Meiji period.

Although for a time he captured public attention, his combination of Christian Socialism, trade unionism, and Confucian morality had only limited intellectual basis in Japanese tradition. Most intellectuals deserted his cause for Marxism, and the popular movement died with the economic and political difficulties of the post-World War I period.