The Gunners need just one more win from their final two games to clinch a fourth-placed finish in the Premier League following Monday's 3-0 victory over Newcastle United.
With European qualification having looked in doubt after losing to Everton at the start of the month, Mertesacker is pleased with how the side has responded.
"We have struggled heavily with a couple of injuries, but we have regrouped very well and how we managed the situation was maybe more important than last year, so securing Champions League football was another test for us," the German is quoted as saying by Sky Sports News.
"It is not done yet, Everton play Manchester City on Saturday, but we have it in our own hands."
Arsenal play their penultimate game of the season on Sunday when they entertain West Bromwich Albion.