Harry Ellwood McCormick was born on February 28, 1881, in Philadelphia, Pennsylvania. He was a 1904 graduate of Bucknell University, and he began his baseball career with the New York Giants in 1904. In 1904, he played for the Giants and for the Pittsburgh Pirates. He did not play in the major league again until 1908, when he played for the Philadelphia Phillies and the New York Giants.

In 1908, Moose was involved in a play which ultimately cost the Giants the National League pennant. On September 23, 1908, the Giants were playing the Chicago Cubs at the Polo Grounds in New York City. The score was tied at the bottom of the ninth inning at 1-l. Moose was on third base and Fred Merckle was on first when, with two outs, Al Bridwell singled to center field and Moose scored the "winning run". When he saw Moose cross home plate, Merkle ran off the field to the Giant's clubhouse, which was located behind center field, without touching second base. The Cub's second baseman, Johnny Evers, with a baseball in his hand ( whether it was the "game ball" was disputed ), attracted the attention of the umpire, Hank O'Day, and tagged second base. O'Day called Merkle out and declared the game to be a 1-1 tie. The National League president, Harry Pulliam, upheld O'Day's decision. When the game was replayed after the finish of the regular schedule, the Giants and Cubs were tied for first place. the Cubs won the game, the pennant, and the World Series. The play is known as the "Merkle boner".

Moose played for the Giants in 1909, 1912 and 1913. In 1912, he played in the World Series, which the Giants lost to the Boston Red Sox, and again in 1913, when the Giants lost to the Philadelphia Athletics. In both series, Matty was a pitcher and Moose was a pinch-hitter.

In 1914, Moose retired from baseball. He worked in industry and coached baseball at several colleges, including Bucknell. In the late 1940's, he returned to Lewisburg and became director of faculty housing for Bucknell University. He died in Lewisburg on July 9, 1962, and he is buried in the Lewisburg Cemetery.