Giroud then sent the previously despondent Arsenal fans wild when he headed the victor from a corner, Leicester keeper Kasper Schmeichel only managing to turn the ball onto the underside of the bar before it bounced over the line.
Substitute Aaron Ramsey rifled home to level before Giroud - introduced alongside the Wales global - headed in an 85th-minute victor to settle a breathless encounter in Arsenal's favour.
The Gunners were a goal up inside two minutes as Alexandre Lacazette scored on debut with only his second touch of the game - heading in Mohamed Elneny's cross. "I don't think I would have started him", he said.
"You come away from home, score three goals and are almost there and then concede two.bitterly disappointed", he said.
The guests capitalised on the abundant mistakes from Arsenal players and their sloppy defending as Jamie Vardy eventually made it 2-1 from Marc Albrighton's cross around the half-hour mark. He is an intelligent player.
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.
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His skillset also meshed well with that of Tyrod Taylor , a player who, this summer, nearly became an ex-Bill himself. Watkins' future in Buffalo was already in question after the Bills in May declined to pick up his fifth-year option.
Giroud's 85th minute goal came from a cross by Granit Xhaka.
"The goals we conceded, we can work to get rid of that".
"I feel as well that the goals we conceded, we can work together to get rid of that because one was a ball we lost in the build-up and we conceded two goals on corners. We were quite good past year on corners and overall I believe that with a bit of work we can get that out of the system".
The Gunners look strong as they are coming off the Community Shield win over Chelsea at Wembley while Leicester has also played well in preseason training matches.
Arsenal will be looking to improve their Premier League form this season after the disappointment of their fifth-placed finish in the top flight last term.
As the game looked to be slipping away from Arsenal's grasp, Aaron Ramsey equalised for the hosts in the 82nd minute.