Robert A. Lovett, 90, a New York financier who directed the buildup of U.S. air power during World War II, helped win congressional approval of the Marshall Plan for the recovery of Europe in the postwar years and served as secretary of defense during the Korean War, died yesterday at his home in Locust Valley, N.Y. He had cancer.

Mr. Lovett was a leading member of the elite cadre of the nation's financial establishment that has supplied advice and talent at the highest levels of government in Washington for the last half century. Self-effacing and shy of publicity, he made his mark in Washington by his ability to get things done.

Apart from his wartime service, he was instrumental in such key negotiations as those in 1949 that ended the year-long Soviet blockade of ground access to West Berlin and the founding in the same year of the North Atlantic Treaty Organization.

A pilot who flew in combat in World War I and won the Navy Cross, Mr. Lovett recognized the potential of air power before World War II. During that war he was in the old War Department as assistant secretary for air. By the end of the fighting, production of American aircraft had soared to 80,000 planes a year.

"The fact that our air forces achieved their huge expansion in time was due more to Bob Lovett than to any other man," said Robert Patterson, the secretary of war at the time.

With the return of an uncertain peace, Mr. Lovett went back to investment banking. In 1947, Gen. George C. Marshall, the secretary of state, summoned him to Washington for an 18-month stint as his chief deputy. Marshall had been Army chief of staff during the war and had been impressed by Mr. Lovett's work.

At State, it was Mr. Lovett's responsibility to explain to a skeptical Congress the nuts and bolts of Marshall's proposal for rehabilitating a war devastated Europe. Armed with reams of facts and statistics, he made his way almost daily to Capitol Hill to brief congressional committees. The Marshall Plan became an enormous success.

So great was Marshall's regard for Mr. Lovett that in 1950, when the general became secretary of Defense, he agreed to take the job only on condition that President Harry Truman persuade Mr. Lovett to work with him. It was understood that Marshall would serve for one year.

Returning to Washington for the third time, Mr. Lovett became deputy secretary of defense, supervising the country's defense buildup during the Korean War. In 1951, he replaced Marshall as secretary and held that position until 1953.

"Lovett did the pick-and-shovel work. Marshall was the front man," said Forrest Pogue, Marshall's biographer. "He deserves considerable credit. He was never taking any bows, never stealing anyone's thunder, and he wasn't running for anything."

One of Mr. Lovett's concerns was to end the U.S. tendency to neglect national security except in time of war.

"We seem to have had only two throttle positions in the past: wide open when we are at war and tight shut when there is no shooting," he said. "We have to work out a system that will give us a cruising speed. When you have to drive some distance, a good cruising speed is the fastest, safest way to get there."

After the election of Gen. Dwight D. Eisenhower as president in 1952, Mr. Lovett returned to private business. But he continued as an unofficial ambassador of the New York financial establishment to the government in Washington and he acted as an adviser and consultant.

When John F. Kennedy was elected president in 1960, he was prepared to give Mr. Lovett a choice of three cabinet positions: State, Defense or Treasury. Mr. Lovett declined on grounds of ill health. But less than a month after taking office, Kennedy made him an informal consultant on the "structure and operations" of government. Mr. Lovett was among those consulted by the president during the Cuban missile crisis of 1962.

Born in Huntsville, Tex., on Sept. 14, 1895, Mr. Lovett was the son of Judge Robert Scott Lovett, who was then general counsel for the Harriman family's Union Pacific Railroad. Judge Lovett's duties carried him across the length and breadth of the continent, and often his son accompanied him.

Although his early roots were in Texas, Mr. Lovett's education was quintessentially Eastern Establishment. By 1910 the family was living in New York, and his father was president of Union Pacific. Mr. Lovett graduated from the Hill School in Pennslyvania at the head of his class, was elected to Phi Beta Kappa at Yale, where he was a member of Skull and Bones, and spent a year each at Harvard Law School and Harvard Business School.

It was while he was at Yale that Mr. Lovett acquired his initial interest in airplanes. He helped form a unit of Yale pilots that trained in the United States and France and saw action in World War I. Mr. Lovett participated in night air raids over Zeebrugge and Bruges.

Back home after the war, he married Adele Brown, the youngest daughter of James Brown, the senior partner in the banking firm of Brown Brothers and one of his family's neighbors on Long Island. Mr. Lovett began his business career with Brown Brothers.

By 1926, he was a partner in the firm. In 1931 he played an important role in the merger of Brown Brothers and Harriman Brothers into what is now Brown Brothers Harriman & Co. Also instrumental in that merger were the Harriman brothers, E. Roland and W. Averell, and former U.S. senator Prescott Bush (R-Conn.), father of the vice president.

As a representative of Brown Brothers Harriman, Mr. Lovett was assigned to London and the European continent. He became convinced that war would break out and in the late 1930s began taking flying lessons on Long Island. His purpose was to prepare himself for the conflict that he regarded as inevitable.

His resumption of flight training led him to study airplane production. He wrote a report on it and then-secretary of War Henry L. Stimson was so impressed by it that in 1940 he asked its author to become his special assistant. Mr. Lovett later observed that Stimson, Marshall and his wife were the only three people in the world to whom he was unable to say "no."

During his Washington years, Mr. Lovett and his wife lived at the Shoreham Hotel. He kept long office hours and led a modest social life. Instead of politicians, bankers and Washington officials, the Lovetts preferred the company of John O'Hara, Dorothy Parker, Archibald MacLeish and James Thurber.

Mr. Lovett liked airplanes, cars, murder mysteries, the theater and almost all movies. He also said his work was his recreation, noting that a man could just as easily bore himself to death as work himself to death. He detested exercise. "I walk from my desk to my car," he once said. "At the hotel I walk from my car to the elevator. That's enough."

When he left Defense in 1953, Mr. Lovett returned to New York to Brown Brothers Harriman and also became chairman of the executive committee of Union Pacific. In the 1960s, he was one of a triumvirate of Union Pacific executives who launched the company on an ambitious and highly successful diversification program that included land development, mining and exploration, production and marketing of petroleum products.

From 1953 to 1977, he was a director of CBS. He also was a life member emeritus of the Corporation of the Massachusetts Institute of Technology, a former trustee of the Carnegie Institution of Washington and chairman of the board of the George C. Marshall Research Foundation. In recent years he had been in semiretirement as a limited partner of Brown Brothers Harriman.

His wife died on Jan. 4, 1986. A son, Robert S. Lovett II, died in 1984 and a daughter, Evelyn Lovett Brown, died in 1967. Mr. Lovett is survived by seven grandchildren and two great-grandchildren.