He was the fourth and youngest son of Hjalmar Hammarskjöld, Prime Minister of Sweden from 1914 to 1917, and wife Agnes Maria Carolina Hammarskjöld (née Almquist).
Foi o cuarto e máis novo fillo de Hjalmar Hammarskjöld, primeiro ministro de Suecia entre 1914 e 1917, e Agnes Hammarskjöld (née Almquist).