He became Norwegian triple jump champion in 1946, 1947, 1949, 1950, 1951 and 1952. He won the national silver in 1948 and 1954, and the bronze in 1955. He edged out Kaare Strøm and Eugen Haugland, who had greatly dominated the event before the war.[2] In 1949 Nilsen also became national champion in long jump. Here he won the silver medal in 1950, and bronzes in 1946 and 1952.[3]
His personal best triple jump was 15.13 metres, achieved at the 1952 Olympics.[4] His personal best long jump was 7.12 metres, achieved in June 1950 at Bislett stadion.[5]