Joining the Constitutionalist Party, Ståhlberg was elected to the Diet in 1904 and entered the government of the autonomous Grand Duchy of Finland in 1905 but resigned in 1907. From 1908 to 1918 he was professor of administrative law at the University of Helsinki.

Consistently democratic in outlook, he was among the first Finnish politicians to call for universal suffrage. His draft of a republican constitution (1917) became the basis of the actual constitution of 1919. In the reshuffling of the political parties when national independence was achieved, Ståhlberg joined the National Progressive Party. Elected president of the new republic (1919), he set the pattern for his successors by making full use of his constitutional powers. He did his utmost to narrow the gap between “Reds” and “Whites” after the Civil War. At the end of his term (1925) he did not seek reelection, for he thought that the right wing would become more easily reconciled to the republic if he stood aside. He lost the presidential election of 1931 by two votes and that of 1937 by one vote.