After breaking into the first team in the 2012-13 A-League season, Sainsbury formed a formidable partnership alongside Dutch defender Patrick Zwaanswijk. This unit was central in helping the Mariners secure their maiden A-League Championship against Western Sydney Wanderers in the 2013 A-League Grand Final. The Mariners won the game 2–0 in front of 42,102 spectators at the Sydney Football Stadium. Commentator and former Socceroo Mark Bosnich rated Sainsbury as his man of the match. The official award went to Daniel McBreen. Following the game, it was speculated that Sainsbury had attracted serious interest from European clubs including Southampton, Roda JC, Basel and Middlesbrough.
