Jan Sierada was an active member of the Belarusian Socialist Assembly. In 1917 he was elected the chairman of the First All-Belarusian Congress. In 1918 he was elected president of the short-lived Belarusian Democratic Republic. In February 1918 he was a member of the Belarusian delegation together with Simon Rak-Mikhailovsky and Alyaksandr Tsvikievich at the peace talks in Brest-Litovsk.