He threw left-handed, batted right-handed, and was listed as 5 feet 11 inches (1.80 m) tall and 185 pounds (84 kg).

In 1937—despite a frustrating minor league season that saw him lose 17 of 21 decisions in the Class A-1 Southern Association—he was recalled by Pittsburgh in the season's closing weeks and on Sunday, October 3, he made his MLB debut by throwing a complete game victory against the Cincinnati Reds, limiting the Reds to six hits and two earned runs.

Heintzelman lost three seasons (1943–45) in the prime of his career, serving in the United States Army in the 65th Reconnaissance Troop (Mechanized) of the 65th Infantry Division in the European theatre of World War II.

But he walked the bases loaded before he was relieved by Jim Konstanty, and the Yankees tied the contest on an unearned run that scored on an error.

Heintzelman's major-league career ended in August 1952, but he went on to pitch for three more seasons in the Triple-A International League before he retired from the mound.