In the 1921 Norwegian parliamentary election, Kolstad topped the list for the Farmers' Party in Østfold and was elected to the Parliament of Norway and subsequently re-elected in all elections until his death. He served in the Standing Committee on Finance in all his parliamentary career, becoming head of the committee in 1928. He served as secretary for the Farmers' Party's parliamentary group in all his years in Parliament and was elected President of the Odelsting in 1931. In this period, he also headed the Østfold chapter of the Farmer's Party for several years.