Mannio first represented Finland internationally at the 2003 European Youth Olympic Festival in Paris, France, where he won the bronze medal. The next year, he competed in the 2004 World Junior Championships in Grosseto, Italy, placing 6th against boys up to two years older and setting a new personal best of 70.63 metres with the men's 800 gram javelin. In 2005, Mannio's first full year with the men's implement, he claimed a bronze at the European Junior Championships in Kaunas, Lithuania.