Mark Shand, chairman of an elephant conservation group, was in New York for a charity auction at Sotheby's.
The New York Police Department said Shand had arrived with a relative at the Rose Bar in the Gramercy Park Hotel at about 1 a.m. Wednesday.
Shand, 62, went out to smoke a cigarette at about 2:30 a.m. and fell backward, striking his head as he tried to re-enter through a revolving door, according to police. Officers found him lying on the ground, police said.
He was taken to Bellevue Hospital, where he was pronounced dead at 11:25 a.m., police said.
On Thursday, his death was ruled an accident, said Julie Bolcer, a spokeswoman for the medical examiner's office. It was caused by blunt impact head trauma with skull fracture and bleeding in the brain, she said.
Shand was known for his work as a travel writer and conservationist. He was the author of several books including "Elephant Tales" and "River Dog: A Journey Down the Brahmaputra."
British royal officials at Clarence House said in a statement that Camilla, Prince Charles and all of her family members are "utterly devastated by this sudden and tragic loss."
"Mark Shand was a man of extraordinary vitality, a tireless campaigner and conservationist whose incredible work through The Elephant Family and beyond remained his focus right up until his death," the statement said.
Sotheby's said it was "deeply saddened" by the news of Shand's death and it was honored to have helped him raise money for his causes.
The Gramercy Park Hotel, located across the street from its namesake park, carries the intricate woodworking and tapestries of its bars and lounge into the adjacent lobby, where the revolving door deposits guests onto a red carpet monogrammed with the letters GPH.
The storied hotel hosted Humphrey Bogart's 1926 wedding; served as a temporary home for a young John F. Kennedy, the Rolling Stones and U2; and was a hangout for Babe Ruth, Bob Dylan, the Beatles, the Clash and Bob Marley.
On Wednesday, police investigators scoped out security cameras in and around the hotel.
An employee directed foot traffic as guests trickled in, some delivered in high-end SUVs. A half-dozen patrons socialized at Jade, the smaller of the hotel's two bars, as detectives paced the sidewalk outside.
A hotel manager said Thursday that he had no record of Shand staying there.
Camilla and Charles are due to go on a tour of Canada next month.