He threw and batted right-handed, stood 5 feet 10 inches (1.78 m) tall and weighed 160 pounds (73 kg) during his playing career.

Originally signed by his hometown Brooklyn Dodgers during the Second World War, DeMars was selected by the Philadelphia Athletics in the 1947 rule 5 draft after batting .328 with 88 runs batted in for the Class B Asheville Tourists of the Tri-State League – DeMars' best overall season in professional baseball.

He managed in the Baltimore organization through 1968 — working alongside future Major League managers such as Earl Weaver, Joe Altobelli, Darrell Johnson, Jim Frey, Clyde King, Cal Ripken Sr., and Billy Hunter.

DeMars left the Phillies after the 1981 season and coached six more seasons with the Montreal Expos (1982–1984) and Cincinnati Reds (1985–1987), where he was a key advisor to playing manager Pete Rose; Rose called DeMars the best hitting coach with whom he had ever worked.

[2] DeMars remained in baseball and returned to the Phillies as a roving minor league batting instructor during the 1990s.