Mr. Kahn, shown here in 2005, died Feb. 24 at 109. (Courtesy of New York Stock Exchange)

The secret to value investing is holding on to underpriced stocks for a long time until they reach their true value. Few people were in a position to follow through as literally as Irving Kahn, who until his death Feb. 24 at 109 was Wall Street’s oldest stockbroker and presumed to be the world’s oldest active investment professional.

The son of Jewish immigrants from Poland, Mr. Kahn landed his first job as a runner delivering papers and securities on the floor of the New York Stock Exchange a few months before the 1929 stock market crash.

“After one week of working there, I decided to quit because I thought the people were crazy,” Mr. Kahn said in an interview in 2012. “They were running around and screaming at each other during trading hours, and they were like clowns!”

His boss persuaded him to stay in finance, and he became a brokerage assistant at the old exchange firm H. Hentz & Co. One evening, he asked a bookkeeper to show him the company’s profit-and-loss ledgers. His eyes immediately fell on a series of investments that consistently made money and rarely showed a loss. They all belonged to one person: Benjamin Graham.

“I went over to Graham’s offices at the Cotton Exchange on Beaver Street and that was the beginning of my career,” he told Barron’s magazine in 2005.

The strategy of Graham, commonly considered the “father of value investing,” was to buy up large shares of businesses that had long-term growth prospects but were temporarily underpriced.

His investment philosophy influenced hordes of young investors, among them Warren Buffett.

Mr. Kahn sought out Graham and became his full-time teaching assistant at Columbia Business School in the early 1930s. He also contributed statistical material to Graham’s and economist David Dodd’s influential 1934 text, “Security Analysis.”

“I stopped wasting time on what people claimed a stock was worth and started looking at the numbers,” Mr. Kahn told the Daily Beast in 2011. He remained close to Graham until his death in 1976.

Mr. Kahn was a partner at several investment firms, including J.R. Williston & Beane and Abraham & Co., before establishing an investment management and advisory firm in 1978, which is now Kahn Brothers Group, with two of his sons.

The Kahns adhered to Graham’s value methodology. “If we buy something which is generally well thought of by the Street and popular, then we’re probably doing something wrong,” he joked to The Washington Post in 1995.

As of 2014, the principals of Kahn Brothers Group managed nearly $1 billion in assets, according to Andrew Kahn, Mr. Kahn’s grandson. The firm’s largest stock holdings include the pharmaceutical companies Pfizer and Merck, Citigroup bank and the New York Times Co.

During his career, Mr. Kahn saw crashes and recoveries, depressions and recessions, bubbles and bursts. Through it all, he stressed the virtues of long-term trading and discouraged gambling investors, whom he saw as interested only in making a quick buck.

To generate new investment ideas, he said, he read at least two newspapers daily and high-tech scientific journals. “I read no fiction, no mystery stories and no sex novels,” he said in 2010. “So, that leaves a lot of time for science.”

Irving Kahn was born in New York City on Dec. 19, 1905. His father was an electrical-fixtures salesman, and his mother ran a shirtwaist shop in their home. He attended the City College of New York for two years before quitting to join Wall Street.

Mr. Kahn was part of an age­defying family and one of four centenarian siblings; his younger brother, Peter, died at 103, and his older sisters, Helen and Lee, died at ages 109 and 101, respectively.

The Kahns were believed to be the world’s oldest quartet of siblings, and his family’s longevity was studied extensively through the Longevity Genes Project at Albert Einstein College of Medicine in the Bronx. Researchers found all the siblings possessed a gene, abbreviated CETP, that controls cholesterol and protects against Alzheimer’s and heart disease.

Mr. Kahn was a founder and past president of the old New York City Job and Career Center, which provided job skills and career opportunities to thousands of high school students, and a founding member of the New York Society of Security Analysts.

His wife of 65 years, Ruth Perl Kahn, died in 1996. Their son Donald W. Kahn died in January. Survivors include two sons, Thomas Graham Kahn and Alan R. Kahn, both of New York City; seven grandchildren; and eight great-grandchildren.

Andrew Kahn confirmed his grandfather’s death and said Mr. Kahn died at his home in Manhattan. The cause could not immediately be confirmed.

Until recently, Mr. Kahn reported regularly to work as chairman of his family’s firm. He took the bus or, when the weather was favorable, walked the 20 blocks from his Upper East Side home to the firm’s Madison Avenue office until he reached 102.

“One of the nice things about this business is that there’s no mandatory retirement age,” his son Thomas Graham Kahn once joked to the Wall Street Journal. “And you allegedly get wiser as you get older.” Mr. Kahn’s two eldest sons retired before their father in their mid-70s.

At 108, Mr. Kahn continued to have an insatiable curiosity and appetite for Wall Street. “I would pay you if you took it away from me,” he quipped, like a true investor. “I’d try to buy it back.”