Werner Stiller transformed himself from an idealistic champion of communism to a freewheeling avatar of global capitalism. (Courtesy of Kristie Macrakis)

On a snowy night in January 1979, Werner Stiller stepped into his office in Berlin, pried open a safe and grabbed the transit papers that would allow him to travel safely from the city’s communist eastern half to freedom in the west.

He had already sent his wife a goodbye letter stuffed with 10,000 German marks, explaining that she and the children would be safer in East Germany, but acknowledging, “I don’t understand a lot of this myself.”

He had a gun inside his jacket, and inside his luggage was a stack of microfiche film four inches thick — the bulk of a secret document cache that made Mr. Stiller, a case officer with the East German police and intelligence agency known as the Stasi, one of the most notable defectors of the Cold War.

A trained physicist, he worked for seven years with the Ministry for State Security’s foreign intelligence service before turning to the West. He later transformed himself from an idealistic champion of communism to a freewheeling avatar of global capitalism.

Under the name Klaus-Peter Fischer, an alias devised in part by the CIA, the shaggy-haired Mr. Stiller launched a second career as a broker and investment banker, reportedly making millions of dollars for Lehman Brothers and Goldman Sachs before retiring in Budapest, where he died on Dec. 20. He was 69.

His son Andreas Hudson confirmed his death but declined to provide additional details. Mr. Stiller’s death was first reported on March 31 by the German news media.

Mr. Stiller oversaw a group of about 30 foreign and domestic agents as a case officer with the Stasi, where he functioned as an undercover middle man. Agents he recruited captured information on microprocessing, nuclear weapons technology and other scientific advances, then passed it to Mr. Stiller through dead-letter drops at railroad stations. The information was reviewed by his superiors, who combed through documents to search for economic advantages in East Germany and for military secrets that could be passed along to allies at the Soviet KGB.

Mr. Stiller was for many years a model citizen of communist East Germany, where he had been a member of the Free German Youth as a teenager and joined the state Communist Party by 21. His loyalties began to shift by the mid-1970s, according to Kristie Macrakis, a Georgia Tech professor who studies espionage and chronicled Mr. Stiller’s story in the 2008 book “Seduced by Secrets: Inside the Stasi’s Spy-Tech World.”

She said that Mr. Stiller was disillusioned with the repressive politics of East Germany, but also frustrated with a career that had stagnated and with a strait-laced German society that abhorred his lavish, womanizing lifestyle.

An affair with Helga Mischnowski, a dissident working as a waitress near the West German border, led to his break from the spy agency. She helped him make contact with the Federal Intelligence Service (BND), the Stasi’s West German rival, and helped him ferry East German documents to the agency in the lead-up to his 1979 departure.

By then, Mr. Stiller had nearly been caught, confronted by a boss who saw him with Mischnowski and told him “to behave correctly and to think of his family,” according to Macrakis’s book.

He fled the country shortly thereafter, helping Mischnowski and her young son escape through Poland and delivering 20,000 pages of Stasi documents to the BND. The defection roiled the spy agency: 17 of its agents were convicted in the West and at least 15 were called back to East Germany.

Perhaps most significantly, Mr. Stiller helped Western intelligence agencies identify Markus Wolf, the East German spy chief who was known as “the man without a face” for remaining undercover for so long.

“Had he been just a case officer, and if he hadn’t defected, he would be an insignificant figure in history,” Macrakis said of Mr. Stiller. His defection, she said, “shined a light” on East Germany’s massive scientific-intelligence gathering efforts “and also shut things down for a while.”

“They had to recall all the agents or tell them to lay low,” she said, adding that the arrests of agents significantly damaged the Stasi’s recruitment efforts.

Mr. Stiller was born in Wessmar, a town west of Leipzig, on Aug. 24, 1947. His mother was married to a prisoner-of-war when she met Mr. Stiller’s father, a blacksmith, and began an affair that lasted for much of Werner’s youth.

He studied physics at what was then known as the Karl Marx University in Leipzig, a recruitment hot spot for the Stasi, before joining the agency and working as an informant at the Physical Society, a scientific organization in Berlin.

Described in one internal Stasi review as having an “exuberant impulsiveness,” he was married at least five times, beginning with a short marriage to a friend from high school. Survivors include his son Hudson and a daughter, Edina Gade, both from his second marriage.

Mr. Stiller abandoned the two children when he moved to West Germany and then the United States, where he taught himself English and went to business school at Washington University in St. Louis. “The CIA Resettlement Group put a pile of color brochures in front of me and told me to choose a city,” Mr. Stiller told the Baltimore Sun in 1992. “I’d always wanted to see the Mississippi so I chose St. Louis.”

He graduated with an MBA in 1983 and then worked at Goldman Sachs in London, where he rose to become head of continental sales in Europe.

Mr. Stiller was sentenced to death in East Germany following his defection and maintained his cover until 1992, after German reunification and the collapse of the Soviet Union. While working for Lehman Brothers in Frankfurt, his second wife recognized him from a television interview, leading him to reconnect with his children and ultimately out himself in an interview with Der Spiegel.

“We had no clue that he was working for the government in the East,” his son Hudson recalled in a video interview posted to YouTube in 2015. “We thought he was just like the normal dude, doing physics jobs, sometimes a bit late home.”