He switched to modern pentathlon in 1963, following the advice from his PE teacher, joining Újpesti Dózsa.

His first major achievement in the sport came in 1966 when he won a gold medal at the Junior World Championship, in both team and individual events.

[citation needed] Kelemen was the first modern pentathlete to win the individual event at both Junior and Adult World Championships.

From 1981, he was the director of the shooting center at Újpesti Dózsa, and also coached Csilla Füri, another Hungarian modern pentathlete.

He also served as a director of the technical board in the National Modern Pentathlon Association.