In 1936 he became the first Yale swimmer to compete in the Olympic Games. In Berlin in early August, he won a silver medal in the 4×200 m freestyle relay event (August 11, swimming the 2nd leg; USA time 9 minutes and 3.0 seconds) and was fourth in his semi-final of the 400 m freestyle event and did not advance to the 6-man final. Immediately after the Olympics, members of the U.S. swim team, including Macionis, took part in "ad hoc" barnstorming swim meets in Europe. Due to World War II, the 1940 Summer Olympics were canceled, however a committee established by the International Swimming Hall of Fame recognized those U.S. swimmers who would have qualified for the Olympic Team that year. Macionis was one of those so recognized. He is also a member of the Pennsylvania Swimming Hall of Fame in State College, Pennsylvania.