In 1916, he travelled to the United States with Hachishiro Mikami to compete in the U.S. National Championships, marking the first participation of a Japanese in one of the Grand Slam tournaments.

He won the singles title at the Newport Casino Invitational, defeating Bill Johnston, the 1915 U.S. National champion, in the final in five sets.

He continued to play tennis, and advanced to the semi-finals at the 1918 U.S. National Championships, but lost to local favorite and seven-time winner Bill Tilden in three straight sets.

With his partner Seiichiro Kashio in the doubles event, he lost to Oswald Turnbull and Maxwell Woosnam of Great Britain for the gold medal match, 2–6, 7–5, 5–7, 5–7.

[citation needed] Following World War II, Kumagae was asked to coach the Japan National Team, which travelled to Louisville, Kentucky for a tournament in 1951.