Moa was born in Tonga and raised in New Zealand.
He was signed by Hull F.C. in January 2009 and arrived a week before the club's fixture with Huddersfield Giants which he featured off the bench in his debut. Moa is good friends with former Hull F.C. and Tonga teammate Willie Manu and former Hull F.C. and Tonga teammate Epalahame Lauaki (now at Bradford Bulls). The trio were commonly known by the Hull fans as 'The Tongan Mafia'.
Sam signed for the Sydney Roosters for the 2013 NRL season, winning the premiership with them at the end of the season. Many observed that Moa's performance in the grand final was particularly memorable, flattening Manly's Glenn Stewart with an aggressive kick-off return in the 1st minute. Controversially, Moa was not the Churchill Medallist with this being awarded to the Sea Eagles' half-back Daly Cherry-Evans.