 Castrén was born in Turtola and graduated in 1887 as a Bachelor of Law.From 1888 to 1892, he worked in the finance division of the Senate of Finland, and from 1888 to 1898 in the Castrén & Snellman attorneys-at-law office.From 1892 to 1904, Castrén was a member of the board in the Kansallis-Osake-Pankki bank.