Niilo Halonen made his international début at the 1958 World Championships, where he was 14th. At the 1960 Winter Olympics he was a surprising silver medalist behind German Helmut Recknagel. Halonen won his last international championship medal at the 1962 World Championships, a bronze. His last international success came on New Year’s Day in 1964, when he was second at Garmisch-Partenkirchen, the only time he medaled in the Four Hills Tournament. He also took part in the 1964 Winter Olympics, placing 14th on both the Normal and Large Hills. Halonen also won the 1966 Lahti ski jumping event. After his competitive days ended, he worked as a school teacher and ski jumping coach, and also worked with the Finnish Ski Federation, as a manager of the ski jumping team and marketing manager of the federation. In 1981 Halonen was selected as Finnish coach of the year. Since 1992 he has worked as a ski jumping coordinator with FIS and at the 1997 World Championships in Trondheim, Halonen acted as a ski jumping judge.