He was considered by the Soviet regime to be a representative of Soviet Jews. He suffered from poverty and homelessness, and gradually abandoned his religious upbringing in favor of revolutionary activities. After the range of the government repression in 1908, he was sentenced to life settlement in the Irkutsk region.
Dimanstein escaped and left the Russian Empire for France until the March Revolution 1917.
At that time Russia was at war and Dimanstein was a propagandist of a peace treaty. He was one of the editors of Trench Truth (Окопной правды).
Dimanstein played a significant role during the Bolshevik October Revolution in 1917. He was appointed a head of Yevsektsiya in January 1918.
In 1920 Dimanstein was sent to Bukhara People"s Soviet Republic where he established Soviet institutions and supported creation of a local Party-approved elite.
In 1922-1924 Dimanstein worked in the Agitation Department of Ukraine. In 1924 he returned to Moscow where he headed different propaganda departments which aimed to spread Soviet ideology among non-Russian peoples. Dimanstein was an editor of New East and Revolution and Nationality.
He was a steady supporter of Stalin"s policies.
His last appointment was as head of the Central Committee of OZET. He was also editor of the Yiddish language newspaper Der Emes" (The Truth)". Semyon Dimanstein advocated the establishment of the Jewish Autonomous Oblast the Russian Far East.
In 1930 he was against the collectivization of Jewish settlements in Jewish national districts of Southern Ukraine and Northern Crimea. In 1935 Dimanstein was an editor of a propaganda book entitled Yidn in FSSR (Jews in the Soviet Union).
From October 1936, Dimanstein was one of the editors of Forpost, a newspaper in the Jewish Autonomous Oblast"s capital city of Birobidzhan.
On 21 February 1938 Dimanstein was arrested. He received a death sentence on 20 August 1938 and was executed. He was rehabilitated posthumously on 13 August 1955, two years after the death of Stalin.
In political debates, he often clashed with Jewish Socialist Party, Bund and Zionist parties.
In 1904 Dimanstein became a member of the Russian Social Democratic Labour Party in Vilnius.