Danning played for the New York Giants from 1933 through 1942, serving as a backup receiver to Gus Mancuso until 1937 when he became the club's regular catcher. Harry The Horse Danning picked up his nickname from a short story by famed sportswriter Damon Runyan in the 1930s. It stayed with him throughout his 10-year career with the New York Giants.

He caught 801 games, was behind the plate for the Giants in five games in the 1936 and 1937 World Series, and was a member of the National League All-Star teams from 1938 through 1941. In 1939, Danning batted .313 with 16 homers. The next season, he drove in 91 runs. He finished in the top nine in the MVP vote both years. One of his special accomplishments was hitting for the cycle [See cycle] in a game on June 15, 1940 against visiting Philadelphia. What made this cycle more special was an inside-the-park homer.

Three times he hit .300 or better and finished his career with a .285 BA. He earned a reputation (and nickname) for durability. The Horse hit 46 homers and averaged .303 in the period 1937-40. On June 9, 1939 he hit one of 5 home runs that were hit in one inning to break major league record of 4 that was set by Pittsburgh in 1894. He served in the military from 1943 through 1945 [See Military Catchers, but after his discharge, he was unable to resume his baseball career because of arthritic knees. His older brother Ike Danning preceded him to the majors by catching two games for the old St. Louis Browns in 1928.[See BROTHER CATCHERS ]

Jewish athletes are rare in baseball, but Danning joined such greats as Detroit slugger Hank Greenberg and Dodger pitcher Sandy Koufax in the International Jewish Sports Hall of Fame. He was also inducted in the New York Jewish Hall of Fame. [See also JEWISH CATCHERS ].

He spent his declining years in Valparaiso, Indiana where he lived with his daughter after his wife died in 1978. Danning's community involvement included sponsoring the annual Harry Danning softball tournament. One of the diamonds at Valparaiso's Fairgrounds Park is named after him. Although Harry was confined to a wheelchair the last three years of his life, he still made appearances. He generally met with other, older sports enthusiasts at a local Baker's Square for breakfast. He died at age 93 having spent a decade in the majors before going to war to serve his country and then serving his community.