The Germany international was part of the late spending spree manager Arsene Wenger engineered in the summer of that year. Arsenal had begun the season conceding 10 goals in consecutive defeats to Liverpool and Manchester United.
Mertesacker's introduction to life in the English Premier League was hardly auspicious. He was part of the back line that surrendered four goals in a defeat to Blackburn Rovers.
That was ammunition for the critics who cannot quite seem to get over the ex-Werder Bremen skipper's famous lack of pace. That is the common retort of all those who still refuse to acknowledge the impact Mertesacker has had on Arsenal's defence.
But the naysayers either overlook or choose to ignore Mertesacker's true value. He has provided the back four, and the squad as a whole, with a genuine leader.
He is the player the Gunners can count on to rally them during tough moments and stand firm under pressure. A prime example came in Arsenal's recent Capital One Cup victory over West Bromwich Albion.
Wenger trusted a team filled with young charges to negotiate a tricky away tie against an EPL club. But the real source of his confidence would have been the presence of Mertesacker, who stayed steady at the heart of an inexperienced starting 11.
Mertesacker's assured performance at the Hawthornes should have prompted little surprise. For all the sniping he promotes, Mertesacker is usually a feature of Arsenal's best defensive efforts.
After all, he was present for all of the team's league-leading 13 clean sheets last season. That is not pure coincidence or sheer dumb luck.
It is the mark of an accomplished defender who knows how to organize those around him, and a smart leader able to marshal his teammates during the difficult moments.
That leadership is utterly priceless to this current Arsenal squad. Along with Mikel Arteta, Mertesacker has altered the personality of the squad for the better.
Years suffering under the baleful influence of divisive figures like Robin van Persie and Samir Nasri had made Arsenal a fragile team. They were often guilty of losing focus and even surrendering games at the slightest sign of trouble.
But this squad, while not as naturally talented as many of their predecessors, are certainly no pushovers. They are a determined bunch that have scrapped for more than their share of positive results.
Mertesacker exemplifies that mentality more than most as a capable international who is still denied the respect he deserves. He plays with a chip on his shoulder, and that attitude has spread to the likes of Aaron Ramsey and Kieran Gibbs, players hell-bent on getting the accolades they merit.
But it would be churlish to put all of Mertesacker's positives into the psychological bracket. He is a highly skilled defender who rarely lets himself get caught out of position.
His knack for stretching out a leg to intercept a dangerous through pass has become a feature of Arsenal's defending. Mertesacker frequently anticipates the direction of play and puts himself in the way of the most pressing danger.
But not all of Mertesacker's defensive qualities are a mix of the cerebral and stylistic. There is also a rough edge and uncompromising nature to the giant in Arsenal's defence.
He is not loath to putting his foot through the ball and making an uncultured boot into the stands. He has also gradually become accustomed to aggressively attacking ball and man in aerial duels.
Mertesacker has emerged as the dominant defensive presence Wenger has been seeking since he broke up "The Invincibles." That peerless squad relied on the solidity of Sol Campbell at the heart of a stingy defence.
But when Campbell left in 2006, replacing him became one of the most challenging chores of Wenger's era in charge. The Arsenal chief first tried William Gallas.
But fans rarely took to the experienced and cat-quick former Chelsea man. Wenger then bought both Thomas Vermaelen and Laurent Koscielny, hoping one would emerge as the natural linchpin of the back four.
But it was Mertesacker who became the dependable ever-present Arsenal's defence needed. He is not Campbell's equal in pace or muscle, but his impact as a defensive leader has been as significant.
Meanwhile, Vermaelen and Koscielny have been left to compete for the position next to him. As The Daily Mirror's John Cross recently put it:
His partnership with Laurent Koscielny combines the more agile and quick Frenchman and that is undoubtedly the best pairing.
But if Thomas Vermaelen plays, then Mertesacker also gets the nod alongside him. He dovetails around both of them while Vermaelen and Koscielny is a pairing that hasn't always worked so well.
In particular, Mertesacker's combination with the talented but sometimes reckless Koscielny has made Arsenal's defence a team strength.
Mertesacker's value is clearly not lost on Wenger, as Cross also notes:
Wenger admitted in the build-up to West Brom that Mertesacker is irreplaceable. Mertesacker is now becoming a dominant force in defence, said Wenger. He is a recognised centre-back with a leading role in our team.
The "dominant force in defence" Wenger highlights is what Arsenal needed most to get them back into contention. It's why Mertesacker's addition was more important than even that of Arteta, whose early impact on the team's psyche was just as significant.
Santi Cazorla, if he can replicate last season's form, may prove to be as or even more important. Mesut Ozil's mega-money arrival has so far only been significant in altering the club's image and profile.
No, it is Mertesacker who represents Arsenal's best signing since Wenger was forced to start again in 2011. No wonder the Frenchman is eager to award the 29-year-old a new three-year deal, according to The Sunday Mirror's Steve Stammers.
Because Mertesacker has been strengthened by the adversity of injury and consistent criticism from the stands, now many of the squad are following suit.
They are led by the example of a centre-back who was the irreplaceable member of last season's toughest away defence. The defender who has featured in all of the club's record 12 away wins.
Teams don't set records like that with so-called weak links at the heart of their defence. Arsenal owe much of their resolve to the importance of Mertesacker and can ill-afford to be without him.