Arsene Wenger's men survived a testing 90 minutes to secure their eighth win in 10 Premier League outings and continue their charge to finish in the top four
After Steve Sidwell’s early red card, the Gunners struggled at a sun-bathed Craven Cottage but secured the win thanks to Mertesacker’s first-half effort.
The hosts made four changes to the one that collapsed at home to west London rivals Chelsea while for Arsenal, Nacho Monreal and Tomas Rosicky returned in place of English duo Kieran Gibbs and Jack Wilshere.
The early minutes saw the Cottagers struggle to even get a touch of the ball against a confident-looking Arsenal at Craven Cottage.
The visitors possession was almost total and Fulham’s frustration showed when Sidwell earned himself a red card on 12 minutes for an over-the-ball challenge on Mikel Arteta.
The former Chelsea and Aston Villa man was only just making his return from a previous three-match suspension and he really could have no complaints about referee Andre Marriner’s decision.
However, despite that unexpected boost, the Gunners struggled to create openings and on on 20 minutes it was Fulham who went closest to breaking the deadlock.
A surging run from Urby Emanuelson saw the Dutchman dissect the visitors’ backline with a reverse pass that fell perfectly for Dimitar Berbatov. However, the Bulgarian could only direct his close range-effort straight at Wojciech Szczesny.
Indeed, it took until the 27th minute for Mark Schwarzer to be called into action when the veteran Australian made routine saves from first Aaron Ramsey and then Theo Walcott.
Those initial openings were followed by Arsenal’s first real opportunity when Olivier Giroud collected a Santi Cazorla pass well and fired across Schwarzer only to find the outside of the post.
However, Fulham continued to threaten on the counterattack and Emanuelson forced Sczesney into a sprawling save after showing good strength to win the ball back off Monreal.
With two minutes of the half to play, the Gunners finally made their numerical advantage count.
Ruiz was caught sleeping from a Walcott free-kick and that left Laurent Koscielny free to nod across goal for central-defensive partner Mertesacker to stoop in for the opening goal.
The second half started in much the same fashion as the first with a buoyant Arsenal controlling possession without really causing much a threat, though Ruiz scooped a close-range free-kick over the bar for the hosts.
In the glorious spring sunshine, the Gunners looked content to simply pass the ball around Fulham in the second half without exerting real pressure on Mark Schwarzer.
However, the Australian showed good reactions to rush out at the feet of Walcott when he looked set to latch onto a Cazorla pass.
The Gunners’ were almost made to pay for their rather casual approach when Szczesny palmed away a Ruiz free-kick into the path of Stanislav Manolev to tuck away the rebound. However, the Bulgarian’s celebrations were pulled up by a correct offside flag.
In the closing minutes, Giroud earned himself a straight red-card for a poor challenge on Manolev, but Arsenal held out fairly comfortably for the win.