The Mariners were in front with 20 minutes remaining but the match turned on its head within a matter of moments with Mariners veteran Matt Simon sent off and George Blackwood finding an equaliser.
Adelaide’s late heroics mean they will feature in their fourth FFA Cup final when they host Melbourne City in the decider on October 23.
The history books suggested the contest was a mismatch with the cup specialists up against last season’s wooden-spooners, who have not featured in the A-League play-offs since 2014.
But a composed FFA Cup run would indicate the Mariners are a different proposition under former Matildas coach Alen Stajcic.
Impressive young Mariners midfielder Sam Silvera came closest to breaking the deadlock in the opening period, with his header deflected onto the crossbar amid a chaotic few moments in the Adelaide penalty area.
At the other end, Mariners goalkeeper Mark Birighitti was involved in what could have been a pivotal moment when he seemed to lash out at Blackwood, only to avoid sanction from referee Shaun Evans.
Five minutes after the restart it was Adelaide’s turn to strike the frame of the goal, as the Reds’ Norwegian striker Kristian Opseth poked a snap-shot onto the base of the post with Birighitti beaten.
Almost immediately, Jordan Murray could have put the Mariners in front, only for onrushing goalkeeper Isaac Richards to make a smothering save.
There was less good news for Richards on 62 minutes, with the young shotstopper missing a Jack Clisby free-kick from the sideline and allowing the ball to pass unhindered into the goal under pressure from Matt Simon.
But it was Simon who gave Adelaide a significant lifeline by earning a second yellow card to leave his side a man down for the final 18 minutes.
Within two minutes Adelaide were level. Blackwood, having clipped the crossbar with an earlier curling effort, found space in front of goal and drilled home from near the penalty spot, on the back of some deft lead-up play from Nikola Mileusnic.
The Mariners started to look ragged after their first-half enterprise and, with Adelaide a man to the good, it was no surprise when McGree burst through backline to snatch a 90th-minute winner.
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