Every Friday morning, Henry Greenbaum donned a suit and carefully coordinated tie, sometimes a pocket square. His father, decades earlier in Poland, had been a tailor. For more than 50 years, Mr. Greenbaum operated a dry cleaner in Washington. He had about him a gentlemanly elegance, people who knew him recalled — even a “gravitational pull,” said one.

He traveled those Fridays from his home in Maryland to the U.S. Holocaust Memorial Museum in downtown Washington, where he would take a chair at a small desk near the information center and sit from 10 a.m. until 4 p.m., offering a greeting to any visitor who wished to meet him and an answer to anyone, particularly the young, who had a question. “Sick or not sick,” he said, “I go in.”

Mr. Greenbaum, who died Oct. 24 at 90, was a survivor of Auschwitz-Birkenau, the Nazi killing center in German-occupied Poland where more than 1 million Jews, non-Jewish Poles and others were murdered during the Holocaust. At the Holocaust museum, he was one of the longest-serving members of the corps of survivor volunteers, a loyal cohort that translates documents, conducts research and gives tours, making real for visitors the history contained in the museum’s exhibits.

With the passage of time, “we are losing our best teachers about the dangers of unchecked hatred,” Diane Saltzman, the museum’s director of survivor affairs, said in an interview. “We’re seeing a rise in anti-Semitism, and as it is rising, we’re losing our survivors. . . . It’s a very challenging moment, and it’s challenging us to think about what lies ahead when we don’t have the eyewitnesses here to tell us what happened.”

Mr. Greenbaum began his volunteer work in 1994, the year after the museum opened. He traveled around the country speaking to schoolchildren, community organizations and military, law enforcement and other government groups. Hundreds if not thousands of visitors saw or spoke with him at his post in the museum’s atrium, as they entered or emerged from the permanent exhibit presenting the rise of the Nazis and their murder of 6 million Jews.

“We wanted to tell our story the minute we arrived here,” Mr. Greenbaum once told The Washington Post of himself and other victims of the Holocaust. “We promised one another, if you survive, make sure you’ll talk and tell them what they did to us.”

Mr. Greenbaum, shown here with his wife, died Oct. 24 at 90. (Family photo)

He was born Chuna Grynbaum on April 1, 1928, to an Orthodox Jewish family in Starachowice, Poland. The youngest of nine children — six daughters and three sons — he enjoyed what he described as a happy boyhood, playing soccer with his friends and attending the synagogue near the family’s home.

Mr. Greenbaum’s father died shortly before the outbreak of World War II in 1939. In 1940, the family was placed in the Starachowice ghetto. From there, his mother and two sisters and their families were taken to the Treblinka killing center and murdered.

Mr. Greenbaum, who had previously worked in a munitions factory, was sent to a labor camp with three other sisters, all of whom would perish there. One, he learned, was buried in a mass grave he had excavated. Another was killed when she and Mr. Greenbaum attempted to escape by slipping through a hole in the camp’s barbed wire. Mr. Greenbaum was shot in the head during the effort.

“She was the only one I had left,” he said in an oral history with the museum. “She was like my mom, my everything. I had somebody to hang on to. . . . You have somebody there, and all of a sudden, I don’t see her. I started screaming, ‘Faye, Faye, Faye!’ ”

In 1944, at 15, he was transferred to Auschwitz, where he survived the infamous selection of victims to be sent to the gas chamber or to work, and then to Flossenburg, a concentration camp in Germany near the Czech border.

As the Allies closed in, the Nazis subjected Mr. Greenbaum and other surviving prisoners to a death march. On April 25, 1945, “a tank rolled up to us and a skinny little soldier popped his head out of the tank,” Mr. Greenbaum recalled in a speech at Joint Base Andrews outside Washington. “He came up to us, put his hand to his face and said, ‘We are Americans, and you are free.’ ”

By then, Mr. Greenbaum said, he weighed 75 pounds.

After the war, Mr. Greenbaum reunited with a brother. Together, they joined another brother and their last surviving sister in the United States in 1946. Mr. Greenbaum worked at a department store in Washington before joining one of his brothers in opening Windsor Valet in Friendship Heights, which Mr. Greenbaum ran from 1954 until he retired in 1998.

Mr. Greenbaum was a longtime Bethesda, Md., resident. His wife of 63 years, the former Shirley Koperwas, died in 2011. Survivors include their four children, Norman Greenbaum of Clarksburg, Md., Bernard Greenbaum of Rockville, Md., Stanley Greenbaum of Ramsey, N.J., and Gayle Prokopchak of Dickerson, Md.; 12 grandchildren; and eight great-grandchildren.

His death was confirmed by Bernard Greenbaum, who said his father died at an assisted-living facility in Rockville and that the cause was congestive heart failure.

Mr. Greenbaum seemed to find meaning in his work at the museum, particularly his interactions with the young.

“He never wanted to scare or shock people so that they would turn away and not want to listen,” Saltzman said, but neither did he spare them truthful answers to their questions.

For curious schoolchildren, he would patiently roll up his sleeve to show them the tattoo with the number he was assigned at Auschwitz, A18991. “Have you ever heard of Auschwitz?” he would ask.