Anthony Marshall listens to his verdict being read during his trial in New York on Oct. 8, 2009. The son of the late philanthropist and socialite Brooke Astor was convicted of looting his mother's estate while she was suffering from Alzheimer's disease. (Steven Hirsch/New York Post/Reuters)

Anthony D. Marshall, a Marine Corps veteran, diplomat, CIA official and theatrical producer who was convicted of conspiring to steal the wealth of his ailing mother, Brooke Astor, the New York socialite and philanthropist, died Nov. 30 in New York. He was 90.

His lawyer Kenneth E. Warner confirmed the death but did not disclose the cause.

In 2009, Mr. Marshall was convicted of grand larceny and other charges related to the attempted looting of his mother’s assets while she suffered from Alzheimer’s disease. He received a sentence of one to three years in prison but, afflicted by congestive heart failure and Parkinson’s disease, was medically paroled in August 2013 after serving eight weeks.

Mr. Marshall, who did not share the Astor name but was inextricably linked to it, became a central figure in the high-profile legal and family drama surrounding the stewardship of his mother’s health and finances before her death in 2007 at 105. Once the doyenne of the New York elite, Astor became — through painful accusations brought against Mr. Marshall by his son — a prominent name in discussions of the crime known as elder abuse .

In 1953, after two previous marriages, she married Vincent Astor, an heir to the wealth his storied family had amassed in real estate and other ventures. His grandmother hosted the gathering of New York elites known as “the 400.” His father, John Jacob Astor IV, died during the Titanic’s sinking in 1912.

Anthony Marshall, son of Brooke Astor, center, is escorted out of Manhattan District Attorney Robert Morgenthau's office in 2007. (Ramin Talaie/Bloomberg News)

With Vincent’s death in 1959, Brooke Astor found herself in receipt of $60 million for herself and at the helm of another $60 million in a trust created for the alleviation of “human misery.” In the following decades, she rained money — nearly $200 million — on institutions including the Metropolitan Museum of Art, the New York Public Library and the New York Zoological Society, as well as the city’s poor.

Mr. Marshall once engaged a publicist, Vanity Fair reported, to pitch a memoir titled “In the Monument’s Shadow.” (It was never published.)

He was educated at prestigious schools and served with the Marine Corps in the Pacific during World War II, receiving a Purple Heart for wounds sustained at Iwo Jima. During the Cold War, he worked at the CIA on the U2 spy plane initiative and said his assignment, in the mid-1950s, was to travel abroad and obtain permission from foreign leaders to operate the clandestine program from their countries.

Particularly memorable, he said, was his meeting with Iskander Mirza, Pakistan’s first president. “Naturally, I did not tell him what we were doing,” Mr. Marshall remarked in an oral history with the Association for Diplomatic Studies and Training.

“I went to see him and I said . . . we had a special plane that had been adapted to obtain air samples of the atmosphere and could we have permission from his country to do this from an airfield somewhere in the northern part of Pakistan?” Mr. Marshall recalled. “He told me he didn’t see any reason why not.”

Like his mother, Mr. Marshall was reportedly a prolific donor to Republican political campaigns. In the 1970s, he received appointments as ambassador to Madagascar, an Indian Ocean nation where political upheaval helped lead to his withdrawal in 1971, and to Trinidad and Tobago, Kenya and the Seychelles.

In 1982, he produced a short-lived Broadway production of “Alice in Wonderland” starring Kate Burton, the daughter of actor Richard Burton. Later, Mr. Marshall and his wife, Charlene, teamed with veteran producer David Richenthal to produce a revival of the Eugene O’Neill drama “Long Day’s Journey Into Night” in 2003 that featured Brian Dennehy, Vanessa Redgrave and Philip Seymour Hoffman.

Also in 2003 came “I Am My Own Wife,” Doug Wright’s one-man, Pulitzer Prize-winning play that featured actor Jefferson Mays as a German transvestite during the Nazi and communist regimes. Both productions were honored with Tony Awards.

By that time, Mr. Marshall’s mother had begun to show signs of infirmity. In 2006, one of her twin grandsons, Mr. Marshall’s son Philip, a college professor, filed a lawsuit to remove Mr. Marshall as her guardian. He had turned “a blind eye” to his mother, Philip Marshall alleged, “intentionally and repeatedly ignoring her health, safety, personal and household needs, while enriching himself with millions of dollars.”

Among other accusations, the lawsuit charged that Mr. Marshall had neglected to fill medical prescriptions, limited his mother’s caregivers and allowed her to sleep on a couch stained with urine.

“I am shocked and deeply hurt by the allegations against me,” Mr. Marshall said in a statement at the time. “I love my mother, and no one cares more about her than I do. Her well-being, her comfort and her dignity mean everything to me.”

The lawsuit, which was splashed across newspapers and magazines, set off a legal saga involving figures from the highest strata of American society. Henry Kissinger, the former secretary of state and national security adviser, and David Rockefeller, the banker and grandson of industrialist John D. Rockefeller, filed affidavits in support of Philip Marshall’s claims. In later proceedings, actress Whoopi Goldberg, a friend of Anthony Marshall’s, wrote a letter to the court on his behalf.

A settlement was eventually reached in which Annette de la Renta, a friend of Astor’s and the wife of designer Oscar de la Renta, and JPMorgan Chase bank became her guardians. Among other stipulations, Mr. Marshall and his wife resigned as co-executors of Astor’s estate and posted a yacht and Maine vacation home as collateral against other possible claims.

In a subsequent ruling, in December 2006, a judge noted that a court evaluator had found that “allegations of intentional elder abuse . . . were not substantiated.”

The legal drama continued with Mr. Marshall’s criminal trial in 2009, which focused on his handling of his mother’s finances. The prosecution depicted his wife, who was not charged with any crime, as having privately encouraged him to pursue his mother’s wealth.

Mr. Marshall was convicted on 14 of 16 charges, including one count of grand larceny for providing himself a $1 million raise for his assistance with her finances. He was acquitted on a grand larceny charge related to the sale of a Childe Hassam painting for $10 million.

A co-defendant, estate lawyer Francis X. Morrissey Jr., was convicted on conspiracy, fraud and forgery charges. He and Mr. Marshall were sentenced to one to three years in prison. “It is a paradox to me,” the judge said at the time, “that such abundance has led to such incredible sadness.”

Anthony Dryden Marshall was born May 30, 1924, in New York City. His mother, born Roberta Brooke Russell, was the daughter of a Marine Corps commandant.

Anthony was the product of his mother’s first marriage, while still in her teens, to J. Dryden Kuser, a well-to-do Princeton University student whom she met at a dance. Kuser physically abused her, she later recalled. After their divorce, she married Charles “Buddie” Marshall, whose surname her son adopted.

He said that his stepfather limited his contact with his mother, and at 11 he was sent to boarding school. He received a bachelor’s degree in 1950 from Brown University in Rhode Island.

Besides his work on the U2 program, Mr. Marshall’s CIA assignments included a posting in Istanbul. Later, as a diplomat, he was credited with working to increase U.S. commercial interests in Africa, where he also had been a businessman in the 1960s.

His marriage to the former Elizabeth Cryan — the mother of his sons Philip and Alexander — ended in divorce, as did his later marriage to Thelma Hoegnell. In 1992, he wed Charlene Tyler Gilbert, who was previously married to the minister of Astor’s church near the Astor family estate in Maine.

A complete list of survivors could not be confirmed. Mr. Marshall’s lawyer named only his wife, Charlene, three stepchildren and three grandchildren as survivors.

Mr. Marshall said that he saw parallels between his occupations in foreign service and in the arts. “Whether it’s diplomacy or Broadway, it’s all the same thing,” he told the New York Times. “People have conflicts, and they’re expressing them, and you have to work them out.”