He attended the First Middle School but withdrew and took up a career as a writer, supporting himself by working as a teacher and newspaper reporter while turning out works of fiction.
From 1897, he assisted in editing Bocho kaitenshi, a history of the domain of Choshu in the Restoration period compiled by the Mori family, former lords of the domain of Choshu. In 1899 he joined the newspaper called Torozu Choho, where he wrote vigorously in support of gembun itchi, the movement that advocated use of the colloquial language in waiting. In 1903 he founded a magazine called Katei Zasshi, in which he published translations and discussions of the socialist-minded novels of Emile Zola.
In the dark times after Kotoku’s execution in 1911 on charges of having plotted to assassinate the emperor, Sakai founded an advertising and publishing company called Baibunsha, whereby he attempted to provide a livelihood for the members of the socialist movement and keep them from dispersing. In 1917 he ran as a candidate in the general election but lost. In 1922 he participated in the formation of the Japan Communist Party. In 1927 he and Yamakawa Hitoshi began publication of a magazine called Rono (Labor Farmer).
In 1929 he was elected to the Tokyo Municipal Assembly. In 1931 he returned to his home in Fukuoka, where he opened a school for farmers and laborers. The same year, with the outbreak of hostilities between Japan and China in Manchuria, he became chairman of the Antiwar Committee of the Zenkoku Rono Taishuto (National Labor Farmer Populace Party).
On the eve of the Russo-Japanese War, he spoke out against the initiation of hostilities and joined Uehimura Kanzo and KBtoku Shusui in resigning from the Yorozu Chohb because of its prowar policy. He and Kotoku then established the Heiminsha and began publication of a newspaper called Ileimin Shimbun, which carried a translation of the Communist Manifesto done by the two men.