Walsh got his start with firearms shooting clothespins off of his aunt's laundry line. In 1935, he joined the FBI pistol team. Within three years of joining the FBI, he had been presented with two marksmanship trophies from director J. Edgar Hoover. In 1939, at Camp Ritchie, he set the world record in pistol shooting with 198 points out of a possible 200 and won the individual eastern regional pistol championships in 1939 and 1940 and placing second in 1941 after leading for most of the tournament. He placed 12th in the Men's Free Pistol, 50 metres competition at the 1948 Summer Olympics in London. At the 1952 ISSF World Shooting Championships, he won a gold medal with the United States team in the 25 m Center-Fire Pistol event and a silver in the individual version of that event. After winning many tournaments within the United States Marine Corps, he became commander of their marksmanship training, a position that he held for many years, until he retired in 1970. In total, he was selected five times for the All-American Pistol Shooting Team.