[2] He was active in the 1905 Revolution, joining the Bolshevik faction of the Russian Social Democratic Labour Party in 1906.
In 1921, Mantsev received instructions from Lenin to deal with the Popular Socialist economist Alexey Peshekhonov who was working for the People's Commissariat for Agriculture.
From August 1923, a member of the collegium of the People's Commissariat of the Workers 'and Peasants' Inspection of the USSR.
[1] On 7 March 1938 he was called as a witness at the third and largest of the Moscow trials, and made to testify that in 1918 Bukharin and the Left Communists had plotted to overthrow the soviet government and kill Lenin, Stalin and Yakov Sverdlov[6] - an accusation that the communist authorities acknowledged to have been false, after Stalin's death.
[citation needed] Mantsev was sentenced to death on 28 July, and executed on 19 August 1938.