James L. Buckley, a conservative recruit from Connecticut who invaded the New York strongholds of Democrats and liberal Republicans in 1970 and against the odds won a United States Senate seat on the Conservative Party line, died early Friday in Washington. He was 100.

His death, in Sibley Memorial Hospital, resulted from complications of a fall, according to his nephew Christopher Buckley, the author and political satirist.

With his improbable victory, Mr. Buckley became the first third-party candidate to land a seat in the United States Senate since Robert M. LaFollette Jr. of Wisconsin was elected on the Progressive ticket in 1940. He served only one term, from 1971 to 1977, and — although there was an effort to draft him for the Republican presidential nomination in 1976 — never won another election.

But President Ronald Reagan brought him back into public life, appointing him to a State Department post in 1981 and naming him president of Radio Free Europe/Radio Liberty in 1982. In 1985, President Reagan named him to the United States Court of Appeals for the District of Columbia Circuit. Mr. Buckley served as a federal judge for 15 years, the last four as a semiretired senior judge.

The scion of an oil tycoon who left $17 million to each of his 10 children, Mr. Buckley had none of the polysyllabic pyrotechnics of his younger brother William F. Buckley Jr., the conservative author and commentator who founded National Review and hosted the PBS program “Firing Line.” But he was a patient, tenacious voice in a tumultuous era of racial violence, campus unrest and protests against the war in Vietnam.

His opponents in 1970 were liberals in liberal-land: Representative Richard L. Ottinger, a popular three-term Democratic congressman from Westchester County, and the incumbent Republican-Liberal, Senator Charles E. Goodell, who had been appointed in 1968 by Gov. Nelson A. Rockefeller to fill the unexpired term of the assassinated Robert F. Kennedy, a Democrat. Both Mr. Ottinger and Mr. Goodell opposed the war in Vietnam.

Mr. Buckley had lived most of his life in Connecticut and had never held public office; he ran only once, reluctantly and unsuccessfully, as a Conservative martyr against Senator Jacob K. Javits, the esteemed New York liberal Republican, in 1968. And to many New Yorkers he sounded like a carpetbagging ideologue. He supported President Richard M. Nixon and stressed “middle-class values,” a rich man appealing to blue-collar voters with claims that social order was breaking down in America.

But he had many qualities attractive to voters. He was 47, a lawyer and father of six, and he had seen naval combat in World War II. He was athletic and handsome with his brushed Yale crew cut, all-American smile and dimpled cheeks, set off with Ivy League jackets and bow ties. And he had a natural warmth and a charming wit that Nixon could only dream of.

On Election Day in 1970, the political cognoscenti were stunned. Mr. Ottinger and Mr. Goodell split the liberal vote, and Mr. Buckley won with a 38.7 percent plurality.

Analysts called him the architect of a pragmatic new conservatism, combining traditional conservative ideas with common-sense approaches to rising crime, taxes and welfare costs and to deteriorating schools, municipal services and respect for authority.

In the Senate, Mr. Buckley joined the Republicans and generally supported the Nixon administration, although he wanted the Vietnam War to be fought by volunteers and voiced alarm when Nixon announced overtures to Communist China. When it became clear that the Watergate scandal had politically crippled the president, Mr. Buckley publicly urged him to resign.

At the 1976 Republican National Convention, a draft-Buckley movement tried to block Reagan’s momentum. But that movement became moot when President Gerald R. Ford won the nomination. That fall, Mr. Buckley lost a re-election bid to Daniel Patrick Moynihan, a Democrat. Mr. Buckley moved back to Connecticut, and, in 1980, ran for the Senate again, this time as a Republican. He lost to another Democrat, Christopher J. Dodd.

It was his last political hurrah.

James Lane Buckley was born in Manhattan on March 9, 1923, the fourth of 10 children of William and Aloise (Steiner) Buckley. His father was an Irish American lawyer and businessman and his mother a Southerner of Swiss and German descent.

James attended the Millbrook School in Millbrook, N.Y., and Yale University, where he majored in English. After graduating in 1943, he joined the Navy. He took part in the invasions of Leyte, Lingayen and Okinawa and mustered out in 1946 as a lieutenant j.g. He then enrolled at Yale Law School and, after graduating in 1949, practiced law in New Haven, Conn., for several years.

In 1953, he married Ann Frances Cooley. They settled near the family estate in Sharon, in northwest Connecticut, and had five sons and a daughter.

Mr. Buckley, who had been living at an assisted-living facility in Bethesda, Md., is survived by six children, Priscilla, Peter, Jay, William, David and Andrew Buckley; eight grandchildren; and two great-grandchildren. His wife died in 2011.

Mr. Buckey was a naturalist and a bird-watcher — he once even considered ornithology as a profession — and went on two scientific expeditions to the Arctic. But in 1953 he joined the family business, the Catawba Corporation, as vice president and director. He traveled around the world, developing oil and mineral resources and becoming fluent in French and Spanish.

In 1965, he managed William F. Buckley’s Conservative campaign for mayor of New York against the liberal Republican John V. Lindsay and the Democrat Abraham D. Beame. Mr. Lindsay won, but Mr. Buckley received 319,000 votes, a sign of things to come.

In 1968, James Buckley, by then established in New York, drew 1.1 million votes in losing to Senator Javits. His victory two years later was a Conservative Party triumph.

His Senate years were characterized by support for the Nixon administration’s foreign policy, including its plan for gradual withdrawal from Vietnam and a ban on foreign aid to nations that did not cooperate with the United States’ fight against illegal drugs.

After leaving the Senate in 1977, he became an investment banker in New York. But at President Reagan’s call, he served as under secretary of state for security affairs in 1981 and 1982, and as president of Radio Free Europe/Radio Liberty from 1982 to 1985.

His nomination to the federal bench was bumpy. The president wanted him on the appeals court in Manhattan, but the New York bar said Mr. Buckley did not have the requisite experience. His subsequent nomination to the Washington court was confirmed by the Senate over the objections of both Connecticut senators.

His rulings were predictably conservative. In 1992, for example, he held that the government could not give women preferential treatment in awarding broadcast licenses, even though it did so for Black and other minority applicants, because that would deny equal protection of the laws to white men.

He wrote four books: “If Men Were Angels: A View From the Senate” (1975); a memoir, “Gleanings From an Unplanned Life: An Annotated Oral History” (2006); “Freedom at Risk: Reflections on Politics, Liberty, and the State” (2010); and “Saving Congress From Itself: Emancipating the States & Empowering Their People” (2014).

Copies of Mr. Buckley’s last book were sent to every member of the Senate by Chris M. Lantrip, a Dallas businessman and self-described Buckley family devotee.