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.

"Sir Brian Urquhart showed that principles and politics can be blended in practical operations to help prevent and end wars," said Bruce Jones, a former director of the New York University Center on International Cooperation. "That's been the core of the U.N. ever since."

Mr. Urquhart’s U.N. career coincided with the soaring optimism of the immediate post-World War II years, through the growing pains of decolonization that tripled the U.N.’s membership, to the paralyzing years of Cold War ideological confrontation and gridlock.

In 1957, Mr. Urquhart played a pivotal role in setting up the International Atomic Energy Agency, which he called the “first big break in the Cold War” fight over the threat of nuclear annihilation.

Beyond his official duties, Mr. Urquhart performed as the U.N.’s unofficial house historian, defining the public’s understanding of the U.N. through his acclaimed memoir, “A Life in Peace and War,” and extensive essays in the New York Review of Books, where he continued to write into his 90s. In 1986, he retired his self-appointed title as the “U.N.’s oldest inhabitant.”

In addition, Mr. Urquhart wrote biographies of Nobel Peace Prize laureate Ralph Bunche, whom he succeeded as U.N. undersecretary general for special political affairs, and Hammarskjold, whom he considered a personal hero and a model for all secretaries general.

Blunt, urbane and irreverent, Mr. Urquhart possessed an understated sense of humor that showed through even in the face of his multiple brushes with death during peacekeeping efforts. “Better beaten than eaten,” the bloodied U.N. official quipped to a group of reporters in 1961, just moments after surviving a beating by Congolese secessionists in Elisabethville.

Throughout his career, Mr. Urquhart advocated a view that U.N. peacemakers should negotiate with any influential parties, no matter how unsavory. “We don’t have these moralistic hang-ups about whom we can deal with,” Mr. Urquhart told the New York Times in 1981 after having negotiated with the Palestine Liberation Organization to stop firing missiles into Israel from Lebanon.

Mr. Urquhart was one of the only U.N. officials who was accepted by both Israel and the Palestinians as a mediator. In February 1982, a few months before Israel’s invasion of Lebanon, Mr. Urquhart was dispatched to the region on shuttle diplomacy with Prime Minister Menachem Begin — who considered Mr. Urquhart a “man of valor” — and PLO leader Yasser Arafat.

While Mr. Urquhart worked around-the-clock to reach an agreement, he secured a temporary truce, but he eventually realized negotiations were fruitless. “We delivered the PLO as usual,” he told the New York Times, “but Israel took advantage of the cease-fire to occupy the heights around Beirut.”

As a public face of the U.N., Mr. Urquhart drew withering criticism at times. He said he grew to expect hostility from conservative ideologues with ramrod support for Israel in Middle East conflicts. “When the United States gets into Middle East bargaining,” Mr. Urquhart once wrote, “the price of all the rugs tends to go up, and the prospect of a deal becomes more distant.”

Mr. Urquhart also became a target for having worked under Kurt Waldheim, the secretary general from 1972 to 1981 who served in the German army during World War II and was accused of participating in Nazi atrocities.

In 1988, Shirley Hazzard, an Australian-born author and former U.N. official, wrote in the New York Times that Mr. Urquhart had played “an essential role in presenting Mr. Waldheim as a figure of distinction throughout years when verified objections were raised around the world and within the U.N. itself” about his war record.

In his memoir, Mr. Urquhart called Waldheim a “living lie.” By “covering up” his wartime service, Waldheim did “immense damage not only to his own country but to the United Nations and to those who have devoted, and in some cases sacrificed, their lives for it,” Mr. Urquhart wrote.

Brian Edward Urquhart was born in Bridport, England, on Feb. 28, 1919. He was a boy when his father, whom he called “the century’s least successful painter,” abandoned the family. He soon enrolled in an all-girls school where his mother worked as a teacher.

He showed early academic promise, mastering Latin verse at 10 and earning a scholarship to study at the elite Westminster School in London, which prepared him for his later studies at the University of Oxford’s Christ Church college. At the outbreak of World War II, Mr. Urquhart cut short his education in 1939 and enlisted in the British army.

He survived various near-fatal disasters. A minesweeper he was serving in hit a mine in the Thames River. In the summer of 1942, he fell 1,200 feet into a plowed field when his parachute failed to open completely during a training exercise. He spent six months recuperating in a hospital before rejoining his unit. He later saw combat in North Africa and Sicily.

As a 25-year-old chief intelligence officer, Mr. Urquhart warned, to little avail, against Operation Market Garden: the calamitous September 1944 Allied military assault behind German lines in the Netherlands. The event was dramatized in Cornelius Ryan’s 1974 nonfiction book, “A Bridge Too Far,” and the subsequent film version.

Planned by British Field Marshal Bernard Montgomery, Operation Market Garden aimed to seize control of several strategic bridges along the Rhine, paving the way for an Allied advance into Germany.

Mr. Urquhart worried that the British command had grossly underestimated the Germans’ strength and ordered an air reconnaissance mission that revealed the presence of two Panzer divisions near the Allies’ drop-off point.

Mr. Urquhart’s warnings were dismissed by his British superiors. Two days before the planned attack, a medical officer recommended Mr. Urquhart be sent off on medical leave to recover from “nervous exhaustion.” More than 17,000 Allied forces were killed or wounded in Operation Market Garden.

Reflecting on the operation, Mr. Urquhart told the Times in 1982: “The worst way to make an argument is by reason and good information. You must appeal to people’s emotions and to their fears of being made to look ridiculous. I also learned that if you happen, by some chance, to get something right, you become extremely unpopular.”

That experience and his later participation in the liberation of the Bergen-Belsen concentration camp inspired Mr. Urquhart to pursue a career of peacemaking at the United Nations.

After the war, the historian Arnold Toynbee, a family friend, wrote a recommendation for Mr. Urquhart that helped him land a job as an assistant to Gladwyn Jebb, the executive secretary for a committee charged with setting up the United Nations. He later was an adviser for Trygve Lie, the Norwegian who became the U.N.’s first secretary general.

Mr. Urquhart’s first marriage, to Alfreda Huntington, ended in divorce. In 1963, he married Sidney Damrosch Howard Canfield, a daughter of the playwright Sidney Howard and a granddaughter of composer and conductor Walter Damrosch.

In addition to his wife, survivors include three children from his first marriage, Thomas Urquhart of Falmouth, Maine, Katharine Ohno of Brighton, England, and Robert Urquhart of Denver; two children from his second marriage, Rachel Urquhart of Tyringham and Charles Urquhart of Leverett, Mass.; a stepson, Thomas Canfield of Washington; 14 grandchildren; and 10 great-grandchildren.

At the U.N., Mr. Urquhart walked a finely calibrated balance in an organization without sovereign power but that derived strength from accord between its two strongest patrons, the United States and Russia. For the duration of the Cold War, those nations were nuclear antagonists, making progress seem futile.

“Three-fourths of the time, you achieve nothing, but every once in a while, it works just enough to make it worthwhile going on with it,” he said in 1972 in describing the political effectiveness of the U.N. “From day to day, one thinks it hopeless, but cumulatively, it does work.”