The Greek defender completed a move to West Ham last summer after an ill-fated four years in north London.
But Mavropanos has now made his presence felt at the Emirates Stadium after heading home for the Hammers to put them 2-0 up on Thursday night.
West Ham took the lead through Tomas Soucek following a controversial VAR decision, before Arsenal pushed forward in search of an equaliser.
However, it was the 26-year-old who put the game firmly in West Ham's control after heading home from a corner.
While clearly delighted with the effort, Mavrapanos held up his hands in an apologetic fashion as he refused to celebrate the goal in front of his former employers.
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West Ham almost made it 3-0 when Declan Rice gave away a penalty in the final stages, only for Said Benrahma to miss from the spot.
But there's no doubting that the goal could have big consequences.
A victory for Arsenal on Thursday night would have seen them go top of the Premier League table, a position that is now held by Liverpool.
As for West Ham, victory sees them move up to sixth in the Premier League and just four points from the top four.