After the breakup of the Austro-Hungarian Empire, Žerjav was part of the delegation of Austro-Hungarian South Slavs which in December 1918 signed the declaration establishing the Kingdom of Serbs, Croats and Slovenes.
He was elected to the Yugoslav parliament for three consecutive terms, in 1920, 1925, and 1927 (the first time as representative of the Democratic Party, and the last two on the list of the Independent Democratic Party.