On Saturday about 8.30am, officers reacted to a security breach in the grounds of the Queen's Berkshire castle.
A Southampton man, 19, was charged with trespassing on a protected place and possessing an offensive weapon.
The man was arrested and brought to a mental health facility, the Met said.
He has subsequently been sectioned under the Mental Health Act and is being treated by doctors, the force says.
‘Security protocols were alerted within minutes of the man entering the grounds,' said a Scotland Yard spokesperson.
‘A crossbow was found after the man was searched.'
‘Met Police specialised operations are investigating the entire circumstances of this event.'
The Queen spent Christmas in Windsor this year after cancelling her annual trip to Sandringham owing to Covid.
‘We are collaborating with colleagues from the Metropolitan Police on an investigation into this occurrence,' said Superintendent Rebecca Mears.
‘The man was charged with trespassing on a protected place and carrying an offensive weapon. He is currently in detention.
He did not enter any buildings and security measures were activated as soon as he entered the grounds.
‘The Royal Family has been informed.
‘We do not believe the public is in risk.'
The Queen reflected on a year of personal sorrow in her emotional annual Christmas address.
She stated she was missing ‘one familiar chuckle' after Prince Philip died from the coronavirus outbreak.