With the outbreak of the Second World War, the German Army High Command (the OKH) assumed its wartime organisation. Heusinger accompanied the field staff and assisted in the planning of operations for the invasions of Poland, Denmark, Norway, and France and the Low Countries.

Heusinger temporarily assumed his office as Chief of the General Staff of the Army. In this capacity, he attended the meeting at Hitler's Wolf's Lair on 20 July 1944, and he was standing next to Hitler when the bomb exploded.

Heusinger made available all information that he had on the conspirators who had plotted against the Führer. He reaffirmed that he had not participated in the assassination plot since he still felt an obligation to fulfil his duty as a soldier of the German Reich

Heusinger was later appointed head of the West German military from 1957 to 1961 as well as Chairman of the NATO Military Committee from 1961 to 1964.