Lanz entered the Army on 20 June 1914, shortly before the outbreak of World War I and served in the Western Front, and ended it with the rank of lieutenant (Oberleutnant). He was retained in the reduced post-war Reichswehr, being promoted to captain on 1 February 1928. In the period 1932–1934 he commanded a company in an infantry regiment at Gumbinnen, and was subsequently employed in staff duties, being promoted to lieutenant-colonel and Chief of Staff of IX Army Corps on 1 March 1937.