In December 1935, a Washington Post reporter spied “a barelegged young man in a pair of midsummer shorts and a pair of woolen gloves” scampering around a tennis court, “oblivious to the 12-degree-above-zero weather.”

That young man — “finally identified as Allie Ritzenberg, of the tennis-playing Ritzenbergs” — was practicing with his brother Nate for a tournament. For more than eight decades, Allie Ritzenberg remained a fixture of Washington’s tennis world, popularizing the sport and making it chic as an instructor to Jackie Kennedy, George H.W. Bush and countless other members of the city’s elite.

He coached generations of students at Washington’s St. Albans School for Boys and made tennis a democratizing force from the suburbs to the inner city and on State Department tours to Haiti, Libya and other lands.

Mr. Ritzenberg was 100 when he died Nov. 22, at his Bethesda home. He had respiratory failure, said his son, Frederick Ritzenberg.

In 1961, Mr. Ritzenberg began going to the White House to give lessons to the first lady, Jacqueline Kennedy. “She didn’t have the athletic background of some of the Kennedys,” he said, “but she was well coordinated and improved her game considerably.”

Mostly, he held court at the St. Albans Tennis Club, which he established in 1962 at the private boys’ school on the grounds of Washington National Cathedral.

“For a lot of us, he was the ideal of what a coach should be,” one of his St. Albans players, former Washington Post executive Boisfeuillet “Bo” Jones Jr., said Saturday in an interview. “What made him so remarkable was that he didn’t care just about the best players. He cared about anyone who was interested in tennis.”

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One of Mr. Ritzenberg’s first lessons for players of any age was on “court conduct,” or the ethics of the sport: Never walk across another player’s field of vision and, above all, never call an opposing player’s shot out of bounds unless it is clearly outside the line.

He insisted that players at his club wear tennis whites and that, while they were on his court, they leave their titles at the door.

The “George” who summoned Mr. Ritzenberg to South Dakota to play tennis in the middle of a presidential campaign was 1972 Democratic nominee and U.S. Sen. George S. McGovern. The “Bob” who showed up at 7 a.m. on the dot for Mr. Ritzenberg’s first lesson of the day was Defense Secretary Robert S. McNamara. “Kay,” who had a standing midday lesson, was former Post publisher and chief executive Katharine Graham. A white-haired man known on Mr. Ritzenberg’s court as “Earl” was known to another court as Chief Justice Earl Warren.

At one point, there was a 10-year waiting list to join his club, which was among the first in the Washington area to prohibit discrimination based on race, religion or sex.

Several of his proteges, including Harold Solomon, Belmar Gunderson and Nancy Ornstein, became star players, but much of his coaching took place in adult education classes, at inner-city playgrounds or on U.S. military bases overseas.

“I learned how to teach anybody,” he said. “I mean anybody, no matter what age or background.”

Mr. Ritzenberg, who was known for his elegant, flawless style on the court, taught his students to play in a relaxed manner.

“I’ve been told there is a similarity between the kind of tennis I teach and the ballet,” he said in 1975. “The idea is to enjoy the feeling, the movement of the body rather than just enjoying scoring points. I teach pleasure tennis.”

Albert Ritzenberg was born Nov. 11, 1918, in Washington. His father ran a hardware store and later a salvage business. His mother, a homemaker, died in the 1930s.

Mr. Ritzenberg followed two older brothers, Hy and Nate, onto public tennis courts in the District, and the three brothers became known as outstanding players along the East Coast. In 1936, Nate and Allie won the D.C. high school doubles championship, and Nate beat Allie for the singles title.

After winning various local and regional tournaments, Allie Ritzen­­­berg went to the University of Maryland, where he lost only four times before graduating in 1941. He became a teaching professional at the Woodmont Country Club in Rockville, then served in the Army Air Forces in the Pacific during World War II.

After the war, he received a master’s degree in sociology from George Washington University but continued to focus on tennis. He adopted a vegetarian diet while still in his teens.

“I was a restless-youth type, 20 years before that style became popular,” he told Sports Illustrated. “I wore my hair long, ate vegetables, batted around in scruffy clothes. I knew what I really wanted to do was stay with tennis.”

He coached Georgetown’s tennis team for two years in the 1950s, established some of the Washington area’s first indoor courts, working with former Wimbledon champion Pauline Betz Addie, and taught the sport in Haiti, Puerto Rico and Libya.

Mr. Ritzenberg stepped away from teaching and coaching in 2005. As a competitive senior player, he won more than a dozen international championships. He retired from competition as the No. 1-ranked 85-year-old player in the world.

His wife of 72 years, the former Madeleine “Peggy” Snowden, died in 2015. Survivors include four children, Kenneth Ritzenberg of Saratoga Springs, N.Y., Frank Ritzenberg of Reston, Va., Frederick Ritzenberg of Fairfax, Calif., and Katherine Winn-Ritzenberg of Washington; eight grandchildren; and three great-grandchildren.

He often wrote about the sport and had a collection of 3,000 historical and artistic items related to tennis, including the first book with an illustration of tennis from 1530. The collection is now housed at the International Tennis Hall of Fame in Newport, R.I.

Mr. Ritzenberg’s coaching sessions became so popular that they were auctioned off at raffles and political fundraisers.

“You know, after 15 years of teaching someone, they come in as if they were going to a psychiatrist,” he told The Post in 1978, when he charged $35 for a 30-minute tennis lesson. “I tell someone whose husband charges me $75 an hour in legal fees, all he has to be is bright. I’m expected to be bright, cheery, charming, witty and talented.”