Mapusua grew up in Wellington, New Zealand and attended school at Wesley College in Auckland moving south to join Otago for the 2000 provincial season. His strong performances for Otago saw him earn a contract with the Highlanders for the 2002 Super 12 season and he remained a fixture for both Otago and the Highlanders through 2006. Noted for his punishing runs and strong tackling, by the time he left New Zealand he was only the fifth back to have played more than 50 games for the Highlanders.
Mapusua joined London Irish for the 2006–07 Guinness Premiership, and immediately established himself as a top player for the side, scoring 4 tries in 22 matches. He was named the team's top newcomer for his efforts.
In 2008–09, he would have a towering season for London Irish, scoring 9 tries in 27 matches in all competitions and leading the club to the Premiership final. At the conclusion of the season he was named the PRA Players' Player of the Season.
He continued as a regular starter through 2011, and by the conclusion of his stay in London had played 125 games with the side including 91 Premiership matches and 23 Heineken Cup contests.