In 2010, Sainsbury was signed by the Central Coast Mariners along with Sam Gallagher.
Trent and Mariners teammate Marc Warren were selected to spend two weeks at Sheffield United's Academy as part of the two clubs' partnership.
His senior debut came for the Mariners on 24 November 2010 in an F3 Derby draw with Newcastle Jets, with Sainsbury earning praise from coach Graham Arnold for his performance.
Trent went on to make a total of nine appearances in the first team that year.

During the successful 2011–12 campaign, it was announced that Sainsbury had signed a two-year contract extension with the club.
The Mariners went on to win the 2011–12 A-League Premiership.
Trent again made nine league appearances for the club in this season.

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.

In March 2013, Sainsbury was nominated for the NAB Young Footballer of the Year which was eventually won by Marco Rojas.
Sainsbury was named the Mariners Player of the Year for the 2012–13 season.
He was also named in the PFA A-League Team of the Season for 2012–13.

After the announcement of the first A-League All Stars match, Sainsbury was considered one of the main contenders for a place in the squad.
He did not end up playing in the match due to international selection.

Sainsbury scored his first competitive goal for the Mariners on 3 April 2013, in a win over Guizhou Renhe in the 2013 AFC Champions League, his header with under ten minutes remaining proving to be the winner.
His first A-League goal for the side came from a corner in a 2–1 win over Perth Glory in December 2013.
In 2010, Sainsbury was signed by the Central Coast Mariners along with Sam Gallagher. Trent and Mariners teammate Marc Warren were selected to spend two weeks at Sheffield United's Academy as part of the two clubs' partnership. His senior debut came for the Mariners on 24 November 2010 in an F3 Derby draw with Newcastle Jets, with Sainsbury earning praise from coach Graham Arnold for his performance. Trent went on to make a total of nine appearances in the first team that year.
During the successful 2011–12 campaign, it was announced that Sainsbury had signed a two-year contract extension with the club. The Mariners went on to win the 2011–12 A-League Premiership. Trent again made nine league appearances for the club in this season.
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.
In March 2013, Sainsbury was nominated for the NAB Young Footballer of the Year which was eventually won by Marco Rojas. Sainsbury was named the Mariners Player of the Year for the 2012–13 season. He was also named in the PFA A-League Team of the Season for 2012–13.
After the announcement of the first A-League All Stars match, Sainsbury was considered one of the main contenders for a place in the squad. He did not end up playing in the match due to international selection.
Sainsbury scored his first competitive goal for the Mariners on 3 April 2013, in a win over Guizhou Renhe in the 2013 AFC Champions League, his header with under ten minutes remaining proving to be the winner. His first A-League goal for the side came from a corner in a 2–1 win over Perth Glory in December 2013.
Trent Sainsbury