They were the “last prisoners of World War II,” Martha Nierenberg said, the dazzling works of art that had once adorned the home of her grandfather, Baron Mór Lipót Herzog, a Hungarian banker who had assembled one of the finest art collections of its kind in Europe.

The collection included some 2,500 pieces, including sculptures, coins, furnishings and paintings by El Greco, Velázquez, van Dyck, Goya, Courbet, Renoir and Monet. Located on the main avenue of Budapest, her grandfather’s home had the aura of a museum, Mrs. Nierenberg recalled. “I would want to sit down on a chair,” she once told the Associated Press, “and I would hear, ‘Oh, no, you can’t sit down on that chair, that’s 12th century.’ ”

Much of the Herzog collection was looted during the Holocaust amid widespread plunder of Jewish-owned collections across Europe. After reestablishing herself in the United States, Mrs. Nierenberg waged a decades-long legal battle on both sides of the Atlantic to reclaim, on behalf of her family, the works her grandfather had so lovingly collected.

Her campaign, one of the most significant art restitution claims in the wake of World War II, remained unresolved when Mrs. Nierenberg died on June 27 at a senior living facility in Rye, N.Y. She was 96 and had heart ailments, said her daughter Lisa Nierenberg. Mrs. Nierenberg had once accused the Hungarian government of “just waiting for me to give up, to go away, to die.”

Marta Weiss de Csepel was born in Budapest on March 12, 1924, the second of four children. Her father, Alfons Weiss de Csepel, was the son of one of Hungary’s most important industrialists, Manfred Weiss de Csepel, the founder of a metal works that employed tens of thousands of people in the manufacture of planes, trucks, cars, washing machines, pots and pans, cans, and other products.

Her mother, the former Erzsebet Herzog, was a psychiatrist who had studied under Anna Freud. Along with her siblings, she eventually inherited portions of the Herzog art collection after their father’s death in 1934.

Juci, as Mrs. Nierenberg was called, grew up in luxury, in a home that had a tennis court, multiple gardens and a staff that included a butler, maids, a cook, a chauffeur and a governess. The family was Jewish, but she and her siblings were baptized in the Lutheran faith. She attended private and then Presbyterian schools before studying chemistry and physics at a scientific university in Budapest.

World War II began in 1939, and Hungary joined the Axis alliance a year later. On March 19, 1944, the Germans occupied the country, swiftly deporting 440,000 Hungarian Jews. For most of them, the destination was Auschwitz, the Nazi death camp in occupied Poland, where the majority perished in the gas chambers.

After the occupation, Mrs. Nierenberg hid in a hospital under the care of nuns and then with various friends before her safety was secured through what she described as “a very unusual piece of history.”

In a memoir co-written with Nancy Kessler, Mrs. Nierenberg wrote that the Nazis allowed 42 members of her family, herself included, to leave Hungary for safety in Portugal and Switzerland. In exchange, the family’s factories were signed over for 25 years to the Germans, who used them to produce munitions. Her father stayed behind as a hostage.

From Lisbon she made her way to the United States, where her immediate family was eventually reunited and where she pursued graduate studies in organic chemistry at Harvard University and worked in a lab at the Massachusetts Institute of Technology.

In 1951, she married Theodore Nierenberg. Together they partnered with the Danish designer Jens Quistgaard to found Dansk housewares in 1954, an upscale brand that specialized in mid-­century Scandinavian design. The couple built a family of four children, a successful business that they sold in 1985 and a stunning home in Armonk, N.Y. The home was filled with paintings but “no great art,” Mrs. Nierenberg told NPR in 2012.

Her family had sought to safeguard the Herzog collection during the war by hiding it in a factory cellar and in other safe spots. But much of it was destroyed, looted or lost. A portion of the collection, the family came to understand, was delivered to Adolf Eichmann, the Nazi SS official who oversaw the deportation of Hungarian Jews and a chief architect of the Final Solution.

In the chaos of the war and its aftermath, the Herzog collection was scattered across Europe, with pieces ending up in Germany, Russia, Poland, and in a castle and salt mine in Austria. Many works remained in or were returned to Hungary, where they were displayed after the war in museums including the Hungarian National Gallery and the Museum of Fine Arts in Budapest — in some cases with labels identifying them as belonging to the Herzog collection.

The family, including Mrs. Nierenberg’s mother, began seeking to reclaim their artworks as soon as the war ended, at one point placing advertisements in a European art publication seeking information about their whereabouts.

In the 1990s, after the fall of the Iron Curtain, Mrs. Nierenberg undertook private negotiations with the newly democratic Hungarian government. Around 1990, the government returned six paintings and one sculpture of little value, according to Agnes Peresztegi, a lawyer who is of counsel at the Paris-based firm Soffer Avocats and who has assisted Mrs. Nierenberg and her family with their claims for two decades.

But generally, Mrs. Nierenberg told the AP, “they kept stalling and stalling and stalling.”

In 1999, she filed a lawsuit in a Hungarian court seeking the return of 10 (a number later increased to 12) artworks that her mother had inherited.

Hungary returned one of those works, Peresztegi said, and Mrs. Nierenberg seemed to win a major victory in 2000, when a court in Budapest ruled that 10 of the remaining 11 should also be given back.

But the Hungarian government appealed, and in 2008 a court declared that Mrs. Nierenberg was not entitled to restitution of the remaining pieces. At issue, the New York Times reported at the time, were “legal issues as precise as pinpricks” — among them the matter of “prescription,” or whether Hungary had held the artworks long enough to gain ownership of them, and questions involving the statute of limitations on Mrs. Nierenberg’s claim.

In 2010, a nephew of Mrs. Nierenberg’s, Dave de Csepel, became the lead plaintiff in a lawsuit filed in the U.S. District Court for the District of Columbia seeking the return from Hungary to their family of more than 40 artworks with a collective value that reportedly reaches $100 million. The parties are still litigating jurisdiction.

“Hungary . . . continues to benefit from the horror involved in the exploitation of property stolen from Jews and people of Jewish origin during World War II,” Mrs. Nierenberg said in testimony before a U.S. congressional panel in 2000. “In effect, they are ratifying Eichmann’s actions.”

Mrs. Nierenberg’s husband died in 2009. Survivors include their four children, Lisa Nierenberg of Pottersville, N.J., Karin Weisburgh of Larchmont, N.Y., Peter Nierenberg of Steamboat Springs, Colo., and Al Nierenberg of Boxford, Mass.; a sister; 10 grandchildren; and three great-grandchildren.

Mrs. Nierenberg acknowledged that some of the paintings, if returned, would have to be sold to pay her family’s legal fees. But she had identified the place on her wall where she wished for at least one to hang. It was “The Annunciation to Joachim” by the German Renaissance artist Lucas Cranach the Elder, the piece in the Herzog collection most beloved by her mother.

“She never gave up hope that something would happen.” Mrs. Nierenberg told the New York Daily News, “that somehow, maybe not in her lifetime, our family would get the paintings back.”