Pehrsson was active in local and regional politics, and in 1918 became a Member of the Second Chamber of the Riksdag, representing the Liberal Party, but he was not re-elected in 1921.

He switched to the Agrarian Party Bondeförbundet (the "Farmers' League") and was elected a Member of the Second Chamber in 1929, where he served until 1949.

After the Social Democratic cabinet of Per Albin Hansson was overthrown by a parliamentary majority in spring 1936, Pehrsson got the mandate of King Gustaf V to form a non-socialist coalition government.

This turned out to be impossible and Pehrsson briefly headed a one-party cabinet as prime minister from 19 June to 28 September 1936.

After the resignation of the war-time coalition cabinet, Pehrsson-Bramstorp held various important positions in agricultural organisations, while remaining chairman of his party and a member of parliament, until illness forced him to retire in 1949.