Harlu was founded in 1922 after extensive demographic and industrial development in the area. A decision of creating an
Evangelical Lutheran parish to the area had already been made by the
Finnish Senate in 1916 when Finland was still a
Grand Duchy under Russian dominion, and the parish started fully functioning in 1918 in the freshly independent Finland. The
Finnish Orthodox Church didn't establish a parish in the area despite a significant amount of its members residing in the area. Harlu had a land area of approximately 252,6 km² and its population was 7 828 (1939) with a
population density of 30,99 people per km². Harlu constituted of nine villages: Harlu (also Leppäkoski), Heinäjoki, Honkakylä, Häyskynvaara, Joensuu,
Läskelä, Paussu, Tenjärvi and Vehkalahti.