Sopoaga is the oldest among six siblings. His younger brother, Tupou Sopoaga, currently plays for the Cronulla Sharks in the NRL. In his high school days, Sopoaga attended Wellington College and won four local championships with the 1st XV. He captained the side in 2009 and was selected for the NZ Secondary Schools team.
Aged only 19, Sopoaga was selected to represent Wellington in the 2010 ITM Cup and made his debut against Manawatu. He quickly established himself as the squad's starting number 10 with a series of mature performances which belied his age. He finished the season with 88 points in 9 appearances to help Wellington to reach the semi-finals of the competition, where he impressed by scoring 21 points although Wellington fell to eventual champions Canterbury.
Sopoaga followed his former Wellington coach Jamie Joseph south, signing on with the Highlanders for the 2011 Super Rugby season. Expected to back up Colin Slade, he ended up starting the opening game of the season against the Hurricanes with Slade injured, and responded in fine fashion by scoring a try and kicking two penalties in a 14-9 victory. However, in his second start he suffered a shoulder injury which ruled him out until the late stages of the season.