Pelše was born into a peasant family, in "Mazie" farm near Zālīte, Iecava in Bauska District, Latvia to Johan Pelše and his wife Lisa. He was baptized in the village church on March 14 of the same year. As a worker in Riga, Pelše joined the Social-Democratic Party (Bolsheviks) of the Latvian Region in 1915. In 1916, he met Lenin in Switzerland. Between 1914 and 1918, Pelše worked in the workshops of Riga and Vitebsk, as a milling machine operator at the steam-engine making plant in Kharkov, as a punching worker in Petrograd and a loader in the port of Arkhangelsk. On behalf of the local committees he had joined the revolutionary propaganda. He was a delegate of the sixth congress of the Russian Social Democratic Labour Party of the Arkhangelsk party organization. He participated in the February Revolution in 1917 and was a member of the famous Petrograd Soviet. He was actively involved in the preparation and conducting of the October Revolution in 1917. In 1918, he joined the Cheka. In 1918, he was sent by Lenin to Latvia to prosecute the revolution there. In 1919, he was attached to the Red Army and later became a manager in the Construction Ministry of the Latvian Socialist Soviet Republic. After the defeat of the Soviet Latvian government, he returned to Russia in 1919.