19-year-old winger Adisu Bayew earned his first ever A-League start, joining Alessandro Diamanti and Dylan Pierias in the front third.
Meanwhile, fellow 19-year-old Dalibor Markovic started for the first time as a United player, lining up on the left side of a back four.
Andrew Durante was named for his 416th and final national league appearance.
Adisu Bayew had the first chance for the visitors, going on an explosive run before having his shot saved.
United went closer on the quarter-hour mark when Tomi Uskok’s volley from distance was palmed onto the post by Mariners ‘keeper Mark Birighitti.
It was the Mariners who found themselves in front when Jack Clisby fired home the opener following a corner.
The defender was given too much room inside the area, before controlling the ball on his chest and volleying his shot into the bottom corner.
Bayew went close again in the opening 15 minutes of the second half, directing his header on target following a brilliant Diamanti cross, but Birighitti managed to cover ground to deny United an equaliser.
There were shades of Monday night’s loss to Macarthur for United as they looked to find an equaliser in Gosford.
Connor Pain, who came off the bench tried to make things happen in the front third, and almost set up Lachie Wales for the equaliser with 15 minutes remaining. Wales couldn’t make contact with Pain’s cross and the Mariners cleared their lines.
Despite a solid second-half performance from the Green and Black, they were punished by the Mariners on the counter-attack in the dying stages.
Marco Ureña was quick on his feet to tap home the rebound after United ‘keeper Ryan Scott made a strong save.
The win secured third place for Central Coast who will play Macarthur in an Elimination Final next week.
Central Coast Mariners 2 (Clisby 25’, Ureña 95’)
Western United 0