Appearing primarily as an outfielder, Cowan totalled 493 games played, over all or part of eight Major League Baseball (MLB) seasons, for the Chicago Cubs (1963–64), New York Mets (1965), Milwaukee Braves (1965), Philadelphia Phillies (1967), New York Yankees (1969), and California Angels (1969–72).

Cowan threw and batted right-handed, standing 6 feet (1.8 m) tall and weighing 170 pounds (77 kg), during his playing career.

But he struck out 128 times (second in the National League), posted a poor .241 batting average and led the Senior Circuit by committing 11 errors in center field.

Cowan was a utility outfielder, occasional infielder, and pinch hitter for the remainder of his MLB career, and spent two full seasons (1966 and 1968) back in the minors.

He retired after his final big-league game on April 24, 1972, when he struck out as a pinch hitter against Paul Lindblad.