The Frenchman and fellow substitute Aaron Ramsay scored in the final seven minutes as Arsenal twice came from behind, the Welshman levelling two minutes before Giroud headed the decider.
Record signing Alexander Lacazette opened his account within two minutes of the kick-off, but the home side's lead was nearly immediately rubbed out as Shinji Okazaki equalised minutes later before Jamie Vardy put the visitors in front after half an hour.
What they saidArsenal manager Arsene Wenger on Giroud: "I love the man and the player because he's a fantastic guy as well, committed, he loves the club". In the end he made a decision to stay and it was one of the great days of the recent weeks for me. "He has a big weight in the squad and it was a relief for me that he chose to stay".
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. "You saw the desire to get on the end of the corner, terrific". The Algerian was the best player in the Premier League a couple of seasons ago and would've brought something different to the likes of Iwobi, Wilshere and Mesut Özil. The Frenchman can certainly find the back of the net and his effort here led to an entertaining game, although the former Lyon man will be anxious by what he saw at the other end of the pitch.
Lacazette plays for the first time in England's top flight, while Ozil is passed fit to support him alongside Danny Welbeck.
Arsenal's quickfire lead lasted barely three minutes before Marc Albrighton's cross from the left was headed back into the middle by Leicester's new signing Harry Maguire and Okazaki nodded the ball past flailing Arsenal keeper Petr Cech.