In Marseille, he crafted much of Arsenal's best play with clever running and smart distribution though, at times, Arsenal appeared unbalanced in their formation with Jack Wilshere employed on the left side of a front three behind striker Olivier Giroud.
Wenger happy with 'important points'
That gave the French side plenty of space down the right side and Arsene Wenger's team will not be able to afford the same luxuries to upcoming Group F opponents Napoli and Borussia Dortmund.
With Italian side Napoli winning 2-1 against last season's finalists from Germany, this result took on even more importance and the Gunners may need to be tighter in midfield when they host Rafael Benitez's side in their next European match.
Deadline day signing Mesut Ozil made his European debut for the Gunners after an impressive display against Sunderland on Saturday, but he was a flitting influence in the first period.
Walcott had the visitors' best chance before the break when he fired straight at Steve Mandanda despite having his shirt pulled as he bore down on goal, but Marseille should have gone into the break in front.
France international Mathieu Valbeuna was easily the most creative force in the first period and twice went close.
After an uninspiring first period, the game improved in the second as chances began to stack up.
Rod Fanni shot wide just after the interval before Dimitri Payet, finding room on the right wing, had a stinging effort saved by Wojciech Szczesny.
At the other end, Gibbs played a lovely one-two with Ozil before having a shot saved by Mandanda and also saved Mertesacker's embarrassment as the Germany defender returned to the team after illness.
Shortly afterwards, Morel failed to clear Gibbs' cross as Walcott struck his first goal of the season, and when Ramsey signalled his growing influence on the team the victory was secured.
The late penalty did little to dampen Arsenal's delight at the final whistle.
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