It happened in the Chase Bank on East 86th Street, as police say the guard was attempted to escort an unruly customer from the branch.
Police say the dispute began prior to the opening of the bank at 9 a.m.
When the customer arrived early, he was told the bank was not yet open.
Authorities say the customer became agitated and questioned why some people were being let in. He was told they were workers.
When the bank opened, the customer went to the teller and attempted to conduct business.
He reportedly was not happy with the service, told the branch could not complete his transaction. Instead, he was given an 800 number to call, and he then began acting belligerently.
The manager was called over, attempted to calm him down, and after a brief conversation, told the man he had to leave.
The 59-year-old private security guard attempted to walk the man to the door, and a verbal dispute ensued.
It then escalated and became physical, and the guard was stabbed in the neck, police said.
He was rushed to Cornell Hospital in critical condition, but officials said late Friday that he had stabilized.
The suspect fled the scene.
“Our thoughts are with the security guard and his family, and we are cooperating with police,” Chase Bank spokesperson Briana Curran said.
The attacker was described as a Hispanic man with a white shirt and long curly hair.
He fled on foot westbound on East 86th Street.
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Ex-Brit turned Manhattan resident since 2008.