Ichiya Kumagae started his active career in baseball and middle-distance running during high school. He was the first Japanese tennis player to play internationally, competing at the 1913 Far Eastern Games in Manila, where he reached the singles semi-finals and doubles final. In 1915 he won the singles and doubles at the Far Eastern Games in Shanghai. In the following year, he won the singles again, but lost the final in the doubles. Kumagae competed at the US Open in 1916 and played in 60 tournaments in the United States during a three-month tour, ranking #5 in the US that year. After graduating from the Faculty of Finance of the Keio University, he began working for the Bank Department of the Mitsubishi Company and moved to New York. He continued to compete in the United States, ranking #3 in 1919, when he was world-ranked #7. He made the semi-finals at the 1918 US Open, and won the 1919 Great Lakes Championships, defeating Bill Tilden in the final.
Kumagae won two silver medals at the 1920 Olympics, in singles and doubles, partnered by Seiichiro Kashio in the latter, becoming the first Japanese athlete to win an Olympic medal. Kumagae captained the 1921 Japanese Davis Cup team, leading them to the final, where they lost to the United States, still the best finish ever for a Japanese Davis Cup squad through 2020. He was 5-1 in Davis Cup before losing all three of his matches against the United States. In 1922 Kumagae returned to Japan. After World War II Kumagae coached the Japanese national team, when it returned to the Davis Cup in 1951, and wrote a technical manual on tennis in Japanese.
Japan