Kent Cooper, a Washington architect for more than 50 years who played a vital role in the creation of the Vietnam and Korean War memorials on the Mall between the Washington Monument and the Lincoln Memorial, died Jan. 27 at a hospital in the District. He was 91.

The cause was pneumonia, said William Lecky, his former business partner in the architectural firm of Cooper-Lecky Architects.

Mr. Cooper began his career in 1958 as overseer for architect Eero Saarinen in the construction of Washington Dulles International Airport. Over the next four decades, he and his firm designed and remodeled churches in the Washington area, the Visitors Center at Great Falls, Va., station stops for the region’s Metro system, the official residence of the vice president and Blair House, which is the residence for presidential guests across Pennsylvania Avenue from the White House.

Mr. Cooper was best known for his work on the Vietnam Veterans Memorial and the Korean War Veterans Memorial, dedicated in 1982 and 1995, respectively.

For the Vietnam memorial, the basic design was the conception of Maya Lin, a 21-year-old Yale architecture student whose vision of a long, black-granite, open V-shaped wall inscribed with the names of the U.S. military dead has become a national icon. It was chosen in a nationwide competition and has since become one of the country’s most venerated and frequently visited public shrines.

But Lin was not a licensed architect, so the firm of Cooper-Lecky became the architects of record. Lin was given a job in the Cooper-Lecky office, but Mr. Cooper supervised the necessary drawings and site specifications for the project to go forward, and he functioned as frontman in dealing with the federal Fine Arts Commission, which had selected Lin’s winning proposal, Lecky said.

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While sticking to the basic concept of Lin’s design, the commission nevertheless had to deal with veterans’ organizations and patriotic groups who disliked its stark simplicity. They wanted changes, one of which was approved less than a month before the memorial’s dedication: the location of a flagpole and a statue of three battle-weary soldiers.

On the opposite side of the Lincoln Memorial reflecting pool, the Korean War Veterans Memorial also was the subject of a national design competition, which was won by a team of architects from Pennsylvania State University.

But the Penn Staters rebelled at the changes demanded by the Fine Arts Commission and filed a lawsuit, which was unsuccessful. Cooper-Lecky took over the project, producing a design featuring 19 statues of ground troops in windblown ponchos emerging from a grove.

William Kent Cooper was born in Jamestown, N.Y., on Dec. 9, 1926. He was 16 when his father, a business executive, died in an airplane crash. He graduated in 1951 from architecture school at the University of Pennsylvania and received a master’s degree in architecture two years later at the Cranbrook Academy of Art in Bloomfield Hills, Mich.

He married a Cranbrook classmate, Margaret Robb Shook, in 1953. She died in 2012. They had two children, Kate Cooper of London and Manchester, England, and Robert Cooper of Washington; six grandchildren; and two great-grandchildren.

After finishing his work for Saarinen at Dulles Airport, Mr. Cooper opened a firm with Lecky. At its peak, it had 40 architects. The firm closed in 2000, but Mr. Cooper continued to practice independently.

“As one of a handful of architects who midwived architectural modernism into Washington, he helped drag the District’s obsession with antiquated building design kicking and screaming into the light of the 20th century,” the Washington Business Journal wrote of him.

Among Mr. Cooper’s favorite projects, Lecky said, were his work with churches. At Falls Church Episcopal Church, he designed curving pews to enfold the altar, believing that this arrangement would foster a sense of community and unity in the taking of Holy Communion. At St. Mark’s Episcopal Church on Capitol Hill, the pews were replaced with movable chairs, which made it possible to have dances and other social celebrations in the nave.

Mr. Cooper, a resident of Washington, had served on the National Mall Coalition and the Committee of 100 on the Federal City. In 2007, the D.C. chapter of the American Institute of Architects honored him for making a difference in the community.