Mafi represented Tonga U20 in the 2009 Junior World Cup, before moving to Hanazono University in Japan in 2010. After a breakout first season in the Top League for NTT Communications Shining Arcs, Mafi was named in both the Tonga and Japan squad for the 2014 November tests. He opted to play for Japan though, and made an immediate impact which was noted as 'pretty special' by coach Eddie Jones.