After graduation, he went to work for the Grand Duchy, where he served in the government of Turku and Pori Province and in the economic division of the Senate. In 1905, he briefly served as the acting governor of Oulu Province. On 14 February 1910 Rainesalo was named senator. As a senator, he served as the head of the Civil Ministry from 1912-13 during the Markov government and as the head of the House Ministry from 1913 during the Borovitinov government until the October Revolution of 1917. Rainesalo was awarded the rank of Actual State Councillor in 1916 in recognition for his service to the Russian Empire and the emperor.