Lehtimäki was born as a peasant's son in Vahto, a small rural municipality in the province of Southwest Finland.

Lehtimäki's younger brother Hjalmar (1896–1934) was a Red Guard leader in the Finnish Civil War.

After the February Revolution in 1917 Lehtimäki lost his job and started smuggling guns for Finnish revolutionaries.

In the 1918 Finnish Civil War, Lehtimäki fought in the Battle of Vilppula as a commander of a cavalry unit.

Lehtimäki served in the Baltic Fleet as a seaplane pilot and took part on the suppression of the Kronstadt Revolt in March 1921.