Brian E. Urquhart, who became the second official hired by the U.N. organization after its formation in 1945 and who helped shape and manage the international body through the final years of the Cold War, died Jan. 2 at his home in Tyringham, Mass. He was 101.

His daughter Rachel Urquhart confirmed the death but did not provide a specific cause.

A principal adviser to five U.N. secretaries general, Mr. Urquhart (pronounced Er-Kut) played a central role in translating the United Nations’ founding principles into action. He said his work at the U.N. was motivated by “idealism of a very practical kind” following his traumatic experiences in World War II.

Serving in the British military and intelligence, he had witnessed firsthand the slaughter perpetrated by the Nazi regime as well as the vanity of Allied military leaders whose failure to heed obvious cautions led to thousands of preventable casualties.

In the mid-1950s, as the lone official in Secretary General Dag Hammarskjold’s inner circle with military experience, Mr. Urquhart helped invent the practice of U.N. peacekeeping through the establishment of the U.N. Emergency Force, which in 1956 was sent to supervise the cessation of hostilities in the Suez Canal area between Egypt and Israel.

Mr. Urquhart’s greatest legacy was in U.N. peacekeeping, a practice that was not mentioned in the U.N. charter, but that initially involved the deployment of impartial, unarmed or lightly armed soldiers between warring parties to help monitor truces or implement peace agreements. Their blue helmets became a familiar sight in crisis zones around the world.

While Mr. Urquhart spent much of his working life at the U.N. Secretariat in New York, he served as a leading mediator and diplomatic troubleshooter in some of the world’s most truculent conflict zones, including Congo, Cyprus, Kashmir, Namibia and the Middle East.