Morton Sobell, ardent communist operative convicted with Julius and Ethel Rosenberg in the sensational 1951 atomic bomb spy trial, died Dec. 26 in Manhattan. He had steadfastly denied guilt for decades, then abruptly acknowledged complicity in 2008.

His death, at 101, was confirmed by his son, Mark Sobell, who did not cite a specific cause.

Mr. Sobell, who was spared the death penalty imposed on the Rosenbergs, served almost 18 years of a 30-year sentence in federal prison for his role in the Soviet espionage case that gripped the nation. The Rosenbergs were executed in 1953 amid the Cold War anti-communist fervor.

A fourth conspirator, David Greenglass, who was Ethel Rosenberg’s brother, pleaded guilty in exchange for testifying against the Rosenbergs, essentially sealing their deaths. Greenglass served a 10-year prison term and was released in 1960. He died July 1, 2014, at 92.

In his dramatic turnaround confession in September 2008, Mr. Sobell, then 91, admitted in a New York Times interview that he had indeed stolen classified military documents for the Soviet Union near the end of World War II and afterward.

And in a March 2011 Weekly Standard article, he additionally admitted to photocopying hundreds of pages of secret U.S. Air Force documents with Julius Rosenberg and two other men in 1948.

But, he contended, the information involved only artillery and radar devices, not the more critical atomic bomb data that the Rosenbergs were charged with scheming to disclose. This distinction was a factor in prosecutors’ decision not to seek the death penalty against Mr. Sobell.

“This was defensive,” he said. “There’s a big difference between giving that and stuff that could be used to attack our country.”

He also supported earlier claims by researchers that Ethel Rosenberg’s role in the spy ring was greatly exaggerated and Greenglass’s secret notes and sketches on development of the atomic bomb in Los Alamos, N.M., during the war were crude and of little help to the Soviets.

“What he gave them was junk,” Mr. Sobell said.

In a separate interview with the Times in 2001, Greenglass acknowledged that he had magnified Ethel Rosenberg’s activities. He said he falsely asserted at the 1951 espionage conspiracy trial that she had typed handwritten notes from Greenglass for Julius Rosenberg to pass on to Soviet couriers — key testimony that helped lead to her conviction and death sentence.

Mr. Sobell agreed she was only peripherally involved, although she knew what her husband was doing. “What was she guilty of?” he said in his 2008 Times interview. “Of being Julius’s wife.”

Mr. Sobell’s admissions of complicity were a shock to the public. But years earlier, spy case researchers digging into declassified U.S. and Soviet documents had largely agreed the Rosenberg ring members were guilty, though the value of the information was questionable and exaggerated by the government.

“The debate is closed. It’s all over,” Cold War historian Ronald Radosh told The Washington Post in 1997. “Julius Rosenberg was a spy for the Soviet Union, [but] it is clear that the Rosenbergs did not give the Soviets the ‘secret’ of the bomb, and they should not have been executed.”

Morton Sobell was born April 11, 1917, in New York City, the son of Russian immigrants Louis Sobel and the former Rose Pasternack. He graduated from City College of New York in 1938 with a degree in electrical engineering.

He joined the Soviet-aligned Communist Party USA amid the ferment of Depression-era radicalism then rife on many American campuses. Julius Rosenberg was among his classmates and fellow Marxist activists.

In 1939, Mr. Sobell moved to Washington and worked as a civilian at the Navy Bureau of Ordnance as World War II began. He later took a job with the aircraft and marine engineering division of General Electric in Schenectady, N.Y., where prosecutors said he began relaying secret military-related data to Julius Rosenberg.

Like many other specialists assigned to classified work in civilian industry, he was exempt from the Army draft and saw no military duty.

In 1945, he married Helen Levitov Gurewitz. Five years later, as the FBI was closing in on the Rosenbergs in New York, the couple fled to Mexico City with their young son, Mark, and Helen’s daughter, Sydney, by a previous marriage. Using several aliases — Marvin Salt and Morris Sand among them — Mr. Sobell sought passage to Europe, without luck.

When FBI agents learned his whereabouts in August 1950, they notified Mexican authorities, who grabbed him and his family and drove them to the Mexican border at Laredo, Tex., handing them off to waiting U.S. agents.

After his conviction in New York in 1951, both Helen Sobell and Mr. Sobell’s mother, Rose, devoted much of their time lobbying to reverse his conviction and raising money for appeals.

Helen Sobell led picket lines and exhorted support at rallies in the United States and abroad, contending that her husband and the Rosenbergs were railroaded by a government caught up in the Red Scare hysteria of the day.

Rose Sobell gathered more than 8,000 signatures on a petition imploring President Lyndon B. Johnson to pardon her son, to no avail. Philosopher Bertrand Russell and painter Pablo Picasso campaigned for the cause.

Morton and Helen Sobell divorced in 1980. She died in 2002 at 84. Mr. Sobell was married to Nancy Gruber from 1993 until her death in July. In addition to his son, of New York, survivors include a stepdaughter, Sydney Gurewitz Clemens of San Francisco; and three grandsons.

In January 1969, at age 51, Mr. Sobell was paroled after 17 years and nine months in federal prison, five years of it spent in maximum security Alcatraz prison in San Francisco Bay.

He returned to New York City, let his hair grow long and spent time catching up on technological advances in engineering, his profession. He worked in medical electronics.

But he also continued traveling the farther shores of American politics, proclaiming his innocence, booking extensive speaking tours and visiting communist-friendly nations, including Cuba, Vietnam and East Germany.

In 1974, he published an unrepentant account of his prison years, a 525-page tome titled “On Doing Time,” in which he inveighed against the legal “frameup” against him and his fellow spy defendants.

But it was more than that.

“I saw our case as an integral part of the Establishment’s national policy,” he wrote. “Any political trial is used to implement national policy.”

Yet his admission of guilt in the 2011 Weekly Standard interview leaves a man limned with irony — the loyal ideologue who for decades had falsely denied his guilt, only to admit his complicity 20 years after the object of his loyalty had collapsed.

As he told the magazine, “I did it for the Soviet Union.”