The son of an SPD functionary, he became an SPD member in 1910, where he belonged to the party's left wing. In the First World War he was in the Spartacus League and subsequently was a founding member of the KPD. He played a significant role at Stuttgart during the November Revolution. On 9 November 1918 he became Minister of War for the first revolutionary government of the Free People's State of Württemberg. Since the Spartacus League in Stuttgart under the leadership of Fritz Rück and August Thalheimer refused to take part in the government, Schreiner withdrew from the Blos Cabinet on 15 November.