Castrén completed the matriculation examination in 1884 and subsequently the general law degree in 1888, earning the title of vice judge in 1891.

[2][3] Castrén was elected as the legal mayor of Oulu in 1908, a position he held until his resignation in November 1918.

[4] Castrén was first elected as a member of parliament in 1909 and served there until his resignation in 1913 in light of a health condition.

[2] Early on, Arthur Castrén sought to provide the government with strict enforcement powers to suppress internal unrest.

During this time, he issued orders regarding the treatment of Red prisoners, which he apparently later deemed too lenient.