A product of his local club TP-Seinäjoki, Mäntylä joined Premier League club Portsmouth in February 2008, only at the age of 16, signing a three-year contract.[4]
However, during his time in England, he never got any chance to prove his merit and only managed to play in two reserve league matches. After his link with Portsmouth expired, he signed with Inter Turku in August 2010. After the 2011 season his contract with Inter expired, and he signed a two-and-a-half-year contract with Ludogorets.[5] Mäntylä left the team from Razgrad in the summer of 2014 and moved back to Scandinavia. He made his debut in the national team in Helsinki in a friendly game against Czech Republic on 21 May 2014 (2−2).