The German World Cup winner had not started a game all season but was called upon for the final against Chelsea after suspension and injury decimated Arsenal's defence.
Despite concerns over his fitness having only featured for 37 minutes over the course of the entire campaign, Mertesacker was a man-of-the-match contender as Arsene Wenger's side secured a 2-1 win at Wembley.
It was expected the 32-year-old would endure a torrid afternoon against the likes of Diego Costa and Eden Hazard, but instead he was at the top of the game, leading Wenger to single him out for individual praise in his post-match press conference
And Mertesacker said that the more his ability is questioned the better he will become
"I expect myself to perform at that level every single week," he said.
"I don't know if I'm capable but I've done it for 15 years
The more people that write me off, the stronger I get
That's my feeling.
"I think we have shown great support in him (Wenger) as well and for his trust in us
We have shown with that performance, it's a statement."
Alexis Sanchez opened the scoring at Wembley with a controversial goal having appeared to handle the ball in the build-up to his effort - which had originally been chalked out for offside before referee Anthony Taylor overruled his assistant
Chelsea were reduced to 10 men in the second half after Victor Moses was shown a second yellow card for diving inside the Arsenal area - but the newly-crowned Premier League champions would still find a way to equalise through Costa.
The cup was decided when Aaron Ramsey headed home three minutes later.
Mertesacker showed a mixture of composure, strength and leadership to patrol Arsenal's back three in his first start of the campaign.
The experienced centre-back had suffered a serious knee injury in a pre-season match at Lens.
He returned to fitness in early 2017 but could not find a way into the side, with his only appearance coming as a substitute in the final league game of the season against Everton following Laurent Koscielny's red card and an injury to Gabriel.
The former Werder Bremen defender admitted he had "suffered" this season but was delighted to be able to give his all as Arsenal turned a mediocre campaign into a memorable one by securing a record 13th FA Cup win
"I'm just blessed for this opportunity," he added.
"For the trust from my team-mates and my manager because being sidelined for such a long time, I suffered a lot this season because you want to contribute on the pitch.
"To be able to do that today was a very proud moment for myself and to lead the team out for the first time officially as captain is a great moment for myself.
"At the end of the day everyone trusted me and supported me throughout the season and the manager gave me the opportunity."
Source : PA