Heinrichs went to the Swedish co-educational school Nya svenska samskolan. He was one of the Finnish Jaeger troops trained in the volunteer Royal Prussian 27th Jäger Battalion between 1915 and 1918. During the Finnish Civil War he served as a battalion commander in the battles of Tampere and Viipuri. He commanded the III Corps in the Winter War, and from 19 February 1940 the Army of the Isthmus. He was made Chief of the General Staff in June 1940 and promoted to General of Infantry in 1941.