Both sides started well but it was the Mariners with the first chance of the match as Josh Rose was able to have three straight chances that were well saved by Yong-Dae Kim.
A tight first half, saw both teams start to get frustrated at their lack of chances as the physicality ramped up but both teams were kept fairly well in check by some good refereeing.
In the 24th minute the Mariners had a solid opportunity as Nick Montgomery sent the ball in for Matt Simon but he sent his header wide.
Seoul had their first chance just before the half hour mark as a Rafael Costa dos Santos shot was deflected, forcing Liam Reddy in to a good save.
The teams traded chances late in the first half with the Mariners going close two minutes out as Brent Griffiths made space and shot from distance but was denied by a superb Yong-Dae Kim save, which kept it scoreless at the break.
The second half started slowly with both sides looking more determined to avoid conceding rather than scoring goals, with a loss spelling disaster for the side that gave it up.
Just after the hour mark, Reddy came to the rescue to deny Sergio Escudero, who was free at close range and shot, only to be denied by Reddy’s legs.
Il-Lok Yun let the Mariners off the hook as he was left wide open from close range but he sent his header over the bar.
Reddy continued to be an imposing force for the Mariners and in the 67th minute saved his team again as Yo-Han Ko found space and shot powerfully but was denied by another stunning Reddy save.
The Mariners tried pushing through the final ten minutes as they went on the attack with Josh Rose and Bernie Ibini getting forward but struggling to finish their chances.
Disaster struck for the Mariners in the first minute of stoppage time as John Hutchinson struggled to clear a cross and only kicked it in to the back of his own net, giving Seoul the lead.
The result lifts Seoul to the top of the table, from the bottom, while the Mariners now sit outside the places needed to advance and will need a win against Hiroshima and some help in the next match day to advance.