The Spaniard's first goals for the club came minutes apart and just before half-time in Gosford for Josep Gombau's men.
It had been a lacklustre affair before then and the visitors cruised in the second half, with Mariners midfielder Nick Montgomery seeing red in the 70th minute.
The victory saw Adelaide return to second in the table, a point behind leaders Perth Glory, while the Mariners are seventh with one win in seven league games.
Adelaide entered the clash with their coach Gombau linked with a move to J.League side Yokohama F Marinos.
And both teams struggled to make inroads before Sanchez's 40th-minute opener.
Fabio Ferreira's deflected cross from the right fell to Sanchez at the back post and the former Lugo forward headed in a simple goal.
The visitors were 2-0 ahead before the break, albeit perhaps undeservedly.
Tarek Elrich surged into the area down the left before his cross was only partially cleared, but Sanchez was on hand to finish into the roof of the net.
Montgomery's late tackle saw him shown a second yellow card in a blow to the Mariners, whose coach Phil Moss is set to be under increasing pressure.
The Mariners are winless in six league matches ahead of a trip to Newcastle, while Adelaide face rivals Melbourne Victory on Friday.
