Raul Barrera was charged with murder on Monday and ordered held without bail.
Police say Barrera surrendered at 2:30 Sunday morning and admitted he had just killed 23-year-old Sarah Coit.
Neighbors say they heard Coit scream during the attack, and police say Barrera was covered in her blood when he turned himself in.
"She was screaming, 'No, no, stop, stop!' witness Briana Andalore said. "And there was all of [these] things going on. And when we got in at 2:38 a.m., there were police everywhere."
Andalore was spending the night at a friend's apartment at 63 Clinton Street, where Coit shared an apartment with Barrera on an upper floor.
Police say Barrera repeatedly stabbed Coit and left her to die in a pool of blood. When EMS crews arrived she was still alive, but severely injured.
"We didn't know what was going on. We didn't think it was so severe," Andalore said. "So we looked out of the window and her whole head was like off. It was so horrifying."
Coit was rushed to Beth Israel Hospital, where she was pronounced dead.
Police spent much of the day collecting evidence from inside the apartment.
Police say Coit had called police on one occasion in January to complain of a verbal dispute with Barrera when the couple lived in midtown Manhattan.
Police say Coit is from Greenwich, Conn., and worked as a salesperson in Manhattan; Barrera worked in public relations.