Heinrichs 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 Finnish III Army Corps in the Winter War and from February 19, 1940 the Army of the Karelian Isthmus. He was made Chief of the General Staff in June 1940 and promoted to General of Infantry in 1941.
During the Continuation War he commanded the Karelian Army until January 1942, after which he was again appointed the Chief of the General Staff. After the war he served as the Army's commander-in-chief but was forced to resign because of the so called weapon concealment affair.
In 1944 Heinrichs became the second person to receive the Mannerheim Cross, First Class.
Heinrichs was one of the military experts of the delegation sent by president Paasikivi to Moscow for the Agreement of Friendship, Cooperation, and Mutual Assistance