The body was found in Edmonton after police were called just after 1pm by residents who had seen a man armed with a knife.
British media reported that the woman had been beheaded. While police would not confirm that, they did rule out any terrorist connection to the killing.
A 25-year-old man has been arrested on suspicion of murder and is in custody.
Detective Chief Inspector John Sandlin of Scotland Yard said officers were not looking for anyone else over the incident.
"This was a highly visible attack in broad daylight on a residential street," he added.
"Whilst it is too early to speculate on what the motive behind this attack was, I am confident based on the information currently available to me that it is not terrorist related."