Born in Manhattan, New York City, and raised in nearby Mount Vernon, Singleton played both baseball and basketball in high school. He also played baseball in the Bronx Federation League at Macombs Dam Park, across the street from Yankee Stadium. Singleton was drafted out of Hofstra University by the New York Mets as the third overall pick in the 1st round of the 1967 Major League Baseball draft. He made his major league debut with the Mets on June 24, 1970 at the age of 23. In April 1972, he was part of a package traded to the Montreal Expos for Rusty Staub.