Hjalmarson became private secretary to prime minister Arvid Lindman from 1929 to 1930.

He became a member of the Parliament of Sweden's second chamber from 1947, after Professor Gösta Bagge.

He became the leader of the Rightist Party, following the retirement of the farmer and estate-owner Fritiof Domö, in 1950.

After his time as party leader, he served as governor of Gävleborg County from 1963 to 1971.

Later in his life, he became a vocal proponent for limiting the use of prisons, which he believed should only be used for the most dangerous criminals.