Siljo was born as Johan Alarik Sjögren in Oulu.

[1] He completed the Oulun Lyseon Lukio upper secondary school in 1907,[2] and started studies in the University of Helsinki at the same year, but never graduated, instead he focused on writing.

[1] Siljo also worked as an editor in the newspaper Helsingin Sanomat and the periodical Valvoja.

[1] Siljo was on the side of the White Guards in the Finnish Civil War.

He was wounded and captured by the Red Guards in a battle in Orivesi.