UN Secretary-General Dag Hammarskjöld, who in September 1961 died in a plane crash in Africa, became to most of his successors and many more people all over the world a role model in moral leadership. Both in his work and in his speeches he promoted an attitude of international service. He perceived the quest for maturity and maturity of mind as basic elements of this attitude. This article explores Hammarskjöld’s search for maturity in three aspects: first in his capacity as an international civil servant; then the personal quest of his inner person; and finally in his role as moral leader. As an explorative case study, the article traces Hammarskjöld’s main thoughts and insights in response to his mission to Beijing and its follow-up in 1955. It further illustrates the relevance of his approach in the world we live in today.