Police say the 37-year-old employee was stabbed once in the left arm and once in the forehead with a knife at the company’s West 14th Street location in lower Manhattan.
The suspect ran away into the nearby subway station after the incident on Friday evening.
The injured security guard was taken to the hospital in a serious but stable condition.
“I saw him come out on the stretcher and I was like ouch,” an eyewitness told NBC New York.
“Everybody was in a big commotion, crying, everyone was in shock.”
And another witness added: “Screaming and people pushing and shoving … I saw him when they took him out in a stretcher, two paramedics followed by about five or six policemen.”
The store was closed for more than two hours as employees spoke with police officers.
Cleaning company workers then got rid of a trail of blood that was left at the scene.
The investigation remains ongoing, but no one has been arrested in connection to the assault as of Saturday morning, said police.
While there is no universal indoor mask mandate in New York City, individual companies and business owners may require a face covering inside their stores.
Apple’s website states that masks are required inside its stores for all employees and customers.
New York state has seen more than 2.46m Covid-19 cases during the pandemic, and 55,203 deaths.