The man, whose name wasn't immediately released, was outside the headquarters at 120 W. 14th St., near Seventh Avenue, when a man punched him in the back of the head, knocking him to the ground about 2:12 p.m., NYPD officials said.
The attacker, who was with a woman who acted as a lookout, then knocked the victim unconscious with a kick to the face, police said.
"He was out. He was on the ground, mouth gaping open," said a Salvation Army worker who only gave his name as Sam.
The woman seemed shocked by the attack and urged that they flee the scene, Sam said.
"The woman said, 'You better get out of here,'" Sam said.
The attacker fled east on West 14th Street with the woman, police and witnesses said.
The duo, who can be seen in video released by police Wednesday night, hadn't been arrested as of Thursday morning, NYPD officials said.
After the attack, passersby rushed to the man's aid until ambulances could arrive, witnesses said.
"The head was full of blood. He had blood on his clothes," said Nate Randolph, 57, a worker at the Salvation Army.
The bloodied victim was conscious and talking to emergency medical workers as they wheeled him into a ambulance, a witness said.
The victim was treated for a cut to his head at Lenox Hill Hospital, police said.
The attacker had black hair with a red streak in it and was last seen wearing a white T-shirt and beige pants while carrying a black and gray backpack, police said.
The lookout wore a green dress with a hood over her head and black sneakers, police said.
It wasn't immediately clear what sparked the attack.
Salvation Army officials said the three people involved in the attack weren't affiliated with their organization.
"We don't recognize them at all," said Major James Betts. "Our hearts are broken like everyone else's."
Anyone with information should contact Crime Stoppers at (800) 577-TIPS (8477).