In 1917, after the Russian February Revolution, Mickevičius was no longer a fugitive. He arrived from the United States to Petrograd (presently Saint Petersburg) and joined the Russian Social Democratic Labor Party (Bolshevik) (RSDLP(b)). In 1917, he became the editor of Lithuanian socialist (later communist) newspaper "Tiesa". In August 1917, he participated in the 6th congress of RSDLP(b) in Petrograd and supported the creation of the Communist International (Comintern). After the 1917 Russian October Revolution, Mickevičius served as the Commissar of the Soviet government for Lithuanian affairs and was a member of the Central Bureau of the Lithuanian Sections under the Central Committee of the RSDLP(b) and then of the Russian Communist Party (Bolsheviks) (RCP(b)).