Modig began his professional career in the Swedish
Elitserien (SEL) with
Luleå HF during the 2005–06 season. He appeared with Luleå in five seasons through 2010, playing in 126 games. He was drafted by the
Anaheim Ducks in the
2007 NHL Entry Draft, 121st overall in the fourth round. In 2010, the Ducks traded Modig's rights to the
Pittsburgh Penguins in exchange for a sixth-round draft pick in the
2010 draft. The Penguins then agreed to terms with Mattias Modig to a two-year entry-level contract. Modig spent the 2010–11 season with the Penguins'
ECHL affiliate
Wheeling Nailers. However, after only playing nine games, Modig received an
injury to his knee, effectively ending the season. He was slated to play for the Penguins'
American Hockey League (AHL) affiliate
Wilkes-Barre/Scranton Penguins in the 2011–12 season, but the knee injury he had received in the 2010–11 season carried over to the entire 2011–12 season. On 14 June 2012, Modig signed a two-year contract with the
Växjö Lakers of the SEL. In October 2014, Modig was loaned out to the Nottingham Panthers of the EIHL, only to return a week later for the rest of the 2014–15 season. ==International play==