Hassell is the father of Wolf Ulrich von Hassell, who helped the German resistance to Hitler during World War II. As ambassador and deputy head of mission to the United Nations from 1971 to 1978, he oversaw the Federal Republic of Germany regaining its status from observer to full member. His previous diplomatic postings were in the Foreign Office in Bonn, in Belgium and in Italy. He also had a daughter Fey who Was 12 years old when he took up his post in Rome. Between 1933 and 1937, she noted her father‘s reaction to the rise of Nazism and the negative feelings he was unwilling to commit to paper, but which he confided to his family in a diary she kept.