The 2020-21 and 2021-22 winners had the chance to wrap up a first-placed finish against Central Coast Mariners on Saturday, but their coronation was delayed in a 1-1 draw.
That stalemate saw Melbourne City open up an eight-point lead over Adelaide, who needed a maximum against Western to keep the Premiers race alive with two games left to go.
However, the Adelaide dream is now over as they succumbed to a 44th-minute goal from Connor Pain, giving Melbourne City an unassailable seven-point lead at the summit.
On the stroke of half time, Western skipper Joshua Risdon lifted an inswinging cross towards the back post, which was met by Aleksandar Prijovic, who cut back for Pain to tap home from a couple of yards out.
Adelaide struck the post in added time through Lachlan Barr's header, but Western United clung on for a precious three points to confirm Melbourne City's status as Premiers ahead of the Finals series, which will crown the overall champion for the season.
Western United defeated Melbourne City in last year's Grand Final and boosted their hopes of Finals qualification with victory over Adelaide, now sitting eighth in the table and three points adrift of the top six.