They gave the official man-of-the-match award to Alexis Sanchez, scorer of the opening goal, but it was the authority of Mertesacker in snuffing out the attacking threat of champions Chelsea that was crucial to a captivating game.
Victory delivered the Gunners a record 13th FA Cup triumph — and an amazing and unique seventh win for veteran manager Arsene Wenger, who has suffered the slings and arrows of so much criticism this season.
After the final whistle Wenger’s delight was plain to see and when he walked over to raise his arms in front of the Arsenal fans there was nobody jeering now.
He had outwitted his Chelsea counterpart Antonio Conte, and in taking the gamble of restoring Mertesacker to the side Wenger had made a typically astute decision.
The giant German centre-half had not started a match for 13 months due to injury, but had to play here on the big occasion to solve a defensive crisis for the Gunners.
He was simply immense, reminding us that he has a World Cup winner’s medal at home in his cabinet.
Arsenal have now won a record 13 FA Cups
Per Mertesacker marshalled the Arsenal defence expertly
This was a Cup Final to remember — for good and bad
This was a Cup Final to remember — for good and bad.
Victor Moses became the first player to be sent off after a dive in the penalty area, while the goal for Sanchez was shrouded in controversy.
There was also a feast of attacking play from both teams, a cascade of chances — and the best team on the day lifted the trophy thanks to a well crafted winning goal from Aaron Ramsey.
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He was also involved, inadvertently, in the early strike by Sanchez.
Arsenal were pressing every move by Chelsea and when Sanchez closed down an opponent the ball struck his arm and bounced into the box.
Sanchez followed it with alacrity and tucked the ball into the net. Ramsey had been standing well ahead of all the Chelsea defenders and was flagged offside by the linesman.
Arsene Wenger lifts FA Cup as Arsenal players celebrate Sat, May 27, 2017
Arsene Wenger was the centre of attention during the wild Arsenal celebrations
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Arsenal celebrate winning the FA Cup against Chelsea
That brought the curious spectacle of Chelsea protesting for handball and Arsenal protesting that Ramsey was not offside because he wasn’t active as the ball ran past him.
Referee Anthony Taylor consulted his fellow official and awarded the goal, to the joy of Arsenal and the fury of Chelsea.
Was it the correct decision? Probably not, in the cold light of day and judged with the benefit of endless replays.
Never mind, it ignited a wonderful match.
Arsenal were dominant for half an hour, creating several clear chances, the first of them when a sublime passing move put Mesut Ozil free in the penalty area. His chip over the keeper was cleverly stopped on the line by Gary Cahill.
The Gunners were even closer in the 19th minute when Danny Welbeck met a corner from Ozil and glanced his header onto the post. The ball looped up for Aaron Ramsey and he chested the rebound against the post again from an acute angle.
Alexis Sanchez's opening goal was shrouded in controversy
For all the electric intensity of manager Antonio Conte on the touchline, Chelsea were sluggish.
When they finally fashioned a chance striker Diego Costa was foiled by a perfectly judged saving tackle from Mertesacker. It wouldn’t be his last telling intervention.
Another chance swiftly followed for Chelsea as Costa chased a long ball over the top, but this produced a good save from keeper David Ospina.
One problem for Arsenal was the need to play Alex Oxlade-Chamberlain at left wing-back. He was a fish out of water, uncertain of the requirements of the position.
He allowed far too much space to Moses just after the break, but a low shot was well saved by Ospina.
Chelsea now had more of the ball, but Arsenal were a persistent threat the on counter attack.
Arsenal fans were in full voice as their side clinched silverware
The mood changed in the 68th minute with the red card for Moses after his outrageous dive in the box trying to fool the referee into awarding a penalty.
Moses already had one yellow card for a cynical foul on Welbeck and he fully deserved his early bath. At least he didn’t have the effrontery to complain.
Referee Taylor deserves high praise for this decision. The game needs this kind of proper justice.
The 10 men of Chelsea did not lie down. They equalised in the 76th minute thanks to Costa’s strength in the box, chesting down a cross and scoring into the corner.
Wenger responded by introducing Olivier Giroud and straight from the kick off the French striker ran clear on the left and crossed for Ramsey to head into an open net from close range.
What action — and what an atmosphere in the crowd.
At the heart of it all was the unmistakeable figure of Mertesacker, heading out crosses and making intelligent interceptions.
And he had the final act of the drama — to collect the FA Cup and hold it aloft in the grand tradition of the world’s oldest football competition.