After the First World War, the Hassel family was banished from Tanganyika by the British mandate administration and settled in Glücksburg, Schleswig-Holstein.

Drafted into the Wehrmacht armed forces, he served as an interpreter for the Abwehr military intelligence organisation under Admiral Wilhelm Canaris and was decorated with the Iron Cross (2nd Class).

After the war, Hassel joined the newly established Christian Democrat party in the Flensburg district and from 1947 served as mayor of Glücksburg.

[1] He then served in Kurt Georg Kiesinger's cabinet from 1 December 1966 to 21 October 1969 as Minister of Displaced Persons, Refugees, and War Victims.

His son Joachim von Hassel, a naval aviator and Oberleutnant zur See, was killed in a Lockheed F-104 Starfighter crash on 10 March 1970.