Kaplan was born in New York City on March 3, 1941. He studied at Duke University, and earned a bachelor's degree from The New School for Social Research. He later studied at New York University School of Law. In 1963, Kaplan took a job as an economist with the American Stock Exchange, at a salary of $15,000 per year. Kaplan founded the magazine "Institutional Investor" in 1967. He was publisher of the magazine until 1990, and editor-in-chief for two more years, although he sold it in 1984. "The New York Times" reported: "The price was never disclosed but was rumored to be about $75 million."