The goal also continued Lacazette's phenomenal record in the first game of top-flight seasons over recent years - one he might need to add to after Leicester forwards Shinji Okazaki and Jamie Vardy turned the game on its head. Danny Welbeck would bring the Gunners back level with a strike just before the half.
But Leicester, which so astonishingly won the league in 2016, couldn't hold on for its first win at Arsenal since 1973.
The Premier League season started in incredible fashion, with two goals in a matter of minutes after the opening whistle.
The first touch from the 60 million euros ($68 million) signing was at kickoff.
Lacazette, recruited from Lyon, savored the moment, opening his arms out wide to take in the acclamation of his new home.
The current contract Sanchez with Arsenal expires in the summer of 2018.
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.
How could Arsenal line up next season?
How quickly optimism faded as Arsenal's enduring defensive flaws were exposed so soon.
The second half started slowly until Vardy sent another shock through the Emirates with his second goal in the 56th minute.
"It's simple, in every game he is stronger since he joined us", Wenger said.
Leicester couldn't stay on top, though.
Arsenal open the new Premier League campaign on Friday with a home match against 2015-16 champions Leicester City.
Arsene Wenger raised eyebrows last week when he said it was an "ideal situation" having players in their final year at the club.
The Algerian global won the PFA Players' Player of the Year award as Leicester won the title in 2015/16 and looked on the verge of leaving last summer before signing a new four-year deal.