In 1909–12, he studied philosophy at Kraków university, after which he lived illegally, working as a full-time party organiser.

He represented the group at a Bolshevik conference that in Poronino, near Krakow, in September 1913, organised by Vladimir Lenin.

In the new Soviet government, he was appointed chief commissar for Polish Affairs in the People's Commissariat for Nationalities, which was headed by Joseph Stalin.

He was one of a minority of Polish exiles in soviet Russia who backed the decision to pursue the Polish-soviet war in 1920.

In autumn 1923, Lenski rose to prominence in the Polish communist party (KPP) by attacking the triumvirate known as the 'Three W's', whose most senior member was Adolf Warski, for their alleged weakness during the political crisis in Poland that year.

The 'three W's' had been ousted from the leadership as part of the power struggle that began during Lenin's terminal illness because of their links with Leon Trotsky.