Heinz von Westernhagen became a member of the Allgemeine-SS on 1 April 1932 and was assigned to the 1st Sturm of the 17th SS-Standarte. After going back to sea from February to November 1933, he subsequently returned to Germany. Heinz had seen much of the world, including Aus­tralia, and had enjoyed a rich variety of experiences. He composed several articles in which he described his adventures on the seven seas. As well, he wrote interesting and exciting articles about life at sea, some of which were picked up by newspapers. He was promoted to the rank of Sturmmann on 1 November 1933 and to Rottenführer on 1 May 1934. The former sailor was then leading the modest life of a worker at the Phoenix Works in Harburg. On 20 August 1934 he received his promotion to Unterscharführer; at this time he could only serve during his off hours.

Von Westernhagen joined the SS-Verfügungstruppe on 1 October 1934 and was taken on strength by the 1st Company of the Germania Regiment in Hamburg-Veddel. On account of his wide range of experiences and resulting maturity, von Westernhagen was sent to Bad Tolz after only six months. There he successfully passed the second officer training course, and on 1 February 1936 he became a Standartenoberjunker. He subse­quently took part in the obligatory platoon commander training course (10 February to 4 April 1936). Because of his time spent abroad and his maturity, after leaving Bad Tolz, von Westernhagen was transferred to the security service (Sicherheitsdienst) instead of to the SS-Verfügungstruppe. He passed through another course, in Berlin-Grunewald, from April to May 1936, and on 20 April 1936 he was promoted to SS-Untersturmführer. Von Westernhagen and other SS-Untersturmführer were introduced to Adolf Hitler as part of the events marking Reich Party Day 1936

Von Westernhagen worked in the foreign service of Amt III, which frequently took him abroad. He was assigned to Mussolini's chief of police Bochini as liaison officer during the Duce's visit to Berlin in September 1937. Von Westernhagen was promoted to Obersturmführer on 13 September 1937.

1938 saw von Western­hagen in Austria, followed by an extended period in Rome. On 10 Sep­tember 1938 he was transferred to the 1st Company of the SS Regiment Deutschland in Munich; from there he was assigned to the army. From 23 September until 21 December 1938 he was a platoon commander in the 16th Company of the 94th Infantry Regiment. Von Westernhagen returned to the main office of the Security Service on 1 January 1939 and on 30 Jan­uary was promoted to SS-Hauptsturmfuhrer. As criminal investigation work no longer appealed to him in the long run, he pressed for a return to the Verfügungstruppe, especially after the war broke out. Von Westernhagen was assigned to the headquarters staff of the Leibstandarte, and he took part in the fighting in Holland, Belgium and France in 1940 as an acting company commander. Though he wanted to stay with the troops, he was subsequently transferred back to the Security Service and in September 1940 took part in a course on Africa in Rome. He vigorously renewed his efforts to obtain a transfer to a front-line unit, "... so I will not have to feel ashamed to myself and to the world later on."

He was assisted in his efforts by his roommate from Bad Tölz, SS-Hauptsturmführer Richard Schulze, who was serving as a company com­mander with the Leibstandarte, and on 14 March 1941 Heinz von Westernhagen returned to the unit. The campaign in the Balkans began soon afterward and he served for the duration in the staff of the Leibstan­darte's 1st Battalion commanded by Fritz Witt.

On 13 May 1941 Heinz von Westernhagen received the Iron Cross, Second Class. Following the conclusion of the campaign in Greece, in June 1941 the Leibstandarte was transferred into the Wischau-Brünn area in the Protectorate.

At the beginning of the Russian Campaign, von Westernhagen took over the duties of First Special Missions Staff Officer of the Leibstandarte SS Adolf Hitler.

In November 1941 the Leibstandarte attacked Rostov. It took the city but was compelled to withdraw again on orders from above.

The Leibstandarte subsequently occupied a winter position on the Sambek River. After six months of uninterrupted fighting the unit's strength had shrunk to a dangerous level. On 1 June 1942 SS-Hauptsturm-fuhrer Max Wünsche was assigned to the military academy in Berlin; Heinz von Westernhagen succeeded him as Commander of the Assault Gun Battalion. He had already spent some time in the battalion staff, where he had been able to amass some experience with this type of weapon.

In July 1942 the Leibstandarte was transferred to France to rest and refit, while at the same time being reorganized as a panzergrenadier division. The Assault Gun Battalion was organized into three batteries and a headquarters battery. On 10 July 1942, SS-Hauptsturmführer von Western­hagen was awarded the Royal Bulgarian Medal of Bravery IVth Class, 2nd Grade, which was followed on 3 September by the Romanian Crown with Swords without Ribbon Vth Class and on 4 September the Eastern Front Medal. As Commander of the Assault Gun Battalion, on 9 November 1942 he was promoted to SS-Sturmbannführer. His battalion trained intensively in Verneuil, for its next deployment was imminent. His brother Rolf was also serving in the battalion as an assault gun Commander. The period of quiet in France quickly came to an end. In January the division was trans­ferred back to the Soviet Union. Von Westernhagen and his battalion went into action immediately in the Kharkov area, engaging Soviet assault forces which until then had been advancing almost unopposed. The batteries of his battalion were assigned to the Leibstandarte's panzergrenadier units; rarely did von Westernhagen lead his entire battalion into action.

The attack on Kharkov began on 6 March 1943; the Leibstandarte was now on the offensive. Von Westernhagen was decorated with the Irön Cross, First Class and the Tank Battle Badge in Silver. His assault guns took part in numerous attacks. Often the battalion Commander accompanied his batteries. He was always on the move, talking to the Commanders of the panzer-grenadiers, his Company Commanders and gun Commanders. There was no rest for him or any of the others.

The long-planned Operation Zitadelle, which was to cut off the Soviet salient around Kursk, began on 5 July 1943. On the very first day of the attack Heinz was badly wounded in the head.

When he had regained his health, von Westernhagen learned that a new mission awaited him: he was to help build up the heavy panzer battal­ion for the new Ist SS Panzer Corps. As of 5 August 1943 he was the oficial Commander of the lOlst SS Heavy Panzer Battalion. Further treatment of his head wound and subsequent participation in a training course for bat­talion Commanders at the Armored Forces School in Paris delayed his arrival, and he was unable to begin his duties with the battalion until 23 February 1944. The new Commander introduced himself to his men with an impressive inaugural address. He referred to the difficulty of the tasks facing the battalion in the future and appealed to the sense of responsibility in each man. In his speech he made reference to his style of command, which was based on personal, mutual trust between officers, noncommissioned officer and enlisted men: "Here stands no baron before you." As soon as the battalion passed into von Westernhagen's hands the atmosphere changed abruptly. In his humane way, the much-travelled Com­mander established an almost chummy relationship with his men. He was calm and understanding, yet he was the embodiment of the fresh, willing, aggressive front-line soldier. His sometimes downright casual behavior while maintaining his full authority soon won him the affection of his men.