Substitute Aaron Ramsey rifled home the key goal to bring Arsenal level before Giroud - introduced alongside the Wales global - headed in an 85th-minute victor to settle a breathless encounter in Arsenal's favour.
The England global has twice scored scored the opener in this fixture, but is yet to be on the winning team.
The game has, however, come too soon for star forward and contract rebel Alexis Sanchez, who will miss out for the Gunners after he picked up an abdominal strain during Sunday's morning.
New recruit Harry Maguire is expected to be assigned to Lacazette and he is aiming to stop the French forward who has scored 91 goals in the French Ligue 1 over the past four seasons.
Alex Oxlade-Chamberlain and Hector Bellerin start in the wing-back roles with Mohamed Elneny alongside Granit Xhaka in midfield.
Sead Kolasinac also had a snapped shot saved by Kasper Schmeichel, but Leicester were looking unsafe on the counterattack and succeeded in taking the lead in the 29th minute.
They have not been helped by Petr Cech, who's also looked panic-stricken at times.
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Mahrez almost equalized less than two minutes later in sensational fashion, with his volleyed attempt off a set piece going tantalizingly wide of Petr Cech's goal.
While the pundits believed the Gunners were right to celebrate.
He added: "I have plenty of uncertainties about the players who didn't play against Chelsea".
The second half started slowly until Vardy sent another shock through the Emirates with his second goal in the 56th minute.
The impressive Vardy scored his second in the 53rd minute to put Leicester in front once more, curling Riyad Mahrez's corner into the bottom far corner amid poor Arsenal marking. An equalizer by Aaron Ramsey was followed by a header from fellow substitute Giroud sealing a 4-3 victory over the 2016 champions.
But the persistent attack eventually paid off as Ramsey received a cross from Xhaka and had time to take a touch to set up a shot that to level the game a three apiece.
Giroud celebrates the late victor.