Walden received his military education at the Hamina Cadet School and graduated in 1900. He was dismissed from service in 1902, in connection with a conscription strike.
From 20 February 1918, till 5 March 1918, Walden was chief of Vaasa military district. From then till 6 May 1918, he was chief of headquarters of the rear. He then became chief of security of the occupied areas until 22 May 1918. On 28 November 1918, Walden became minister of war and held this position until 15 August 1919. Walden was then promoted to commander in chief of the Finnish army and the Civil Guard (27 November 1918 - 30 December 1918). Walden and general of the White army Carl Gustaf Emil Mannerheim developed a close working relationship that continued throughout the inter war period (head of Finnish Red Cross in the 20th century) and during the Winter War and Continuation War.
After leaving the army, Walden sought a career in business. He moved to Saint Petersburg, the capital of Tsarist Russia, and after few years founded a sales agency for Finnish paper. As a steadfast patriot, Walden also became a prominent figure in the large Finnish community of Saint Petersburg. From 1906 to 1946, Walden worked in the paper industry. He worked to consolidate the Finnish paper industry. He founded Yhtyneet Paperitehtaat Oy (United Paper Mills Ltd) and was the first president of "Finpap", a Finnish paper sales association.
During the Finnish-Soviet Winter War, Walden was recalled to active service. From 3 December 1939 to 27 March 1940, he was a member of the war cabinet. On 27 March 1940 he became Minister of Defence and continued in this role till 27 November 1944. Walden was able to maintain important diplomatic relationships with Sweden and the United States. Walden represented Finland at the Treaty of Tartu of 1920, the Moscow Peace Treaty (1940), and the Armistice treaty of Moscow (1944).
After the Civil war and during the 1930s there was general distrust of the labour movement and the patriotism of the Finnish worker. In Winter War this proved unfounded and the nation was united. Industrial leader Walden and Väinö Tanner (Social Democrat leader and Finnish foreign minister) had developed mutual respect and worked well together. In the so-called Betrothal of January 1940, Finland's industry associations acknowledged the trade unions as negotiation partners regarding labour issues.