Boris was born in a village of Gladkie Vyselki, Mikhailovsk uyezd (county), Ryazan Governorate in a peasant family of Russian Orthodox Old Believers.
During the October Revolution Boris Donskoy was a commissar at the Fort Ino, just outside Saint Petersburg, from where he directed military formations to the Pulkovo Heights.
In spring of 1918 along with Irina Kakhovskaya he went to Ukraine, particularly to the newly established Donetsk–Krivoy Rog Soviet Republic to prepare for fight against the Imperial German occupation.
In April 1918 Donskoy joined the All-Russian Battle Organization of the Left Socialist-Revolutionaries that was sanctioned by its party to carry out an "international terror".
In Kiev, together with Irina Kakhovskaya and G.Smolyansky, Donskoy prepared for an assassination attempt on the commander of the German military governor of Ukraine, Field Marshal Hermann von Eichhorn.