Mattila was drafted in the seventh round by the New York Islanders in the 2006 NHL Draft.
2004-05: Playing for the Rockford Icemen of the USHS Mattila tallied an uncanny 214 points (140 goals, 74 assists) in just 61 games. He was the leading scorer for Illinois High Schools, and for the second straight season he was the leading scorer for the Metro Conference. He was named to the all-state team for the second straight season, and the team MVP for the second straight season. He was also named to the Northwest Metro Conference and H.S. All-Star Teams. He was a member of the 04-05 Illinois Showcase Team and was a participant in Hockey Night in Boston.
2005-06: Rather than finishing his high-school hockey career, Matilla jumped straight to the Jr. level where he played for the NAHL Springfield Jr. Blues. He missed four games due to a shoulder injury, but in the 54 games he played in he finished fourth on the team in scoring with 36 points (14 goals, 22 assists). He had one assist in four games during the NAHL playoffs. After the playoffs were complete, Mattila participated in the Western College Hockey Prospects Tournament where he was the tournaments leading scorer with 11 points (seven goals, four assists) in eight games.
2006-07: Mattila split his second season of junior hockey between the USHL and NAHL. In 36 games for the Chicago Steel, he scored 4 goals with 6 assists and was -4 with 12 PMs. Playing for the NAHL’s Springfield Junior Blues he scored 11 goals, including 5 game-winners, and had 8 assists with 6 PMs.
2007-08: Mattila played a third year of junior hockey with Tri-City in the USHL. In 60 games he scored 13 goals with 17 assists and was -22 with 14 PMs. He scored 6 of his 13 goals on the power play.
2008-09: Mattila appeared in 30 games for Dartmouth as a freshman. He scored 1 goal with 4 assists and was -5 with 2 PMs for a Big Green squad that had a middle-of-the-pack season in ECAC Hockey.
2009-10: Mattila skated in 12 of 32 games for Dartmouth as a sophomore as the Big Green won just ten games and were tied for ninth in ECAC Hockey. He scored 1 goal with 1 assist and was -5 with 2 PMs.
2010-11: Mattila skated in 8 games for Dartmouth as a junior. The Big Green finished third in ECAC Hockey during the regular season, reached the conference semifinals and won the third place game in the post-season but did receive an NCAA bid. Mattila got both of his points, a goal and an assist, in a November game against Clarkson and finished the season -1 with 2 PMs.