2006 saw Mannio's first successes against adult throwers, as he claimed a bronze medal at the 2006 Finnish Championships. However, even though he entered the World Junior Championships in Beijing, China, as the world junior leader, he was convincingly beaten in the final by South Africa's Robert Oosthuizen. The next year, he again failed to claim international gold despite having the best mark coming in; entering the European U23 Championships in Debrecen, Hungary as the only man in the field to have broken 80 metres, Mannio was narrowly pushed out of the medals altogether and had to be content with 4th.