When Tinsley Mortimer came to town

When dashing 33-year-old man-about-town Prince Casimir Sayn-Wittgenstein-Sayn marched into the Cartier Polo with a new blonde on his arm, none of the assembled army of photographers had a clue who she was.

'And then one of the photographers asked her name,' explains Casimir. 'And she said, "Tinsley Mortimer," and then suddenly there was this wall of photographers.'

Tinsley Mortimer, 32, is a New York phenomenon. Routinely described by Vanity Fair as 'social royalty' and hailed as the reigning WASP queen of New York's fashion and charity circuit, Mortimer is a blue-blooded version of Paris Hilton. The long-running drama of her marriage with her now estranged husband Topper (Robert Livingstone Mortimer) constantly put the couple in the public eye, with The New York Times going so far as to describe a night out with the Mortimers as like an evening scripted by Noël Coward.

According to the gossip, friction in the marriage developed as a result of Tinsley's rocketing social career, co-chairing charity benefits such as the Winter Wonderland Ball at the New York Botanical Garden. Topper, a hedge funder and scion of an old-money oil dynasty, talked of the 'frivolous' nature of New York's social scene. 'It's not necessarily the type of goal that anybody should strive for, going out every night for the sake of getting their picture taken. These girls went to good colleges. You would think they'd have something better to do.' Topper has the old-fashioned view that you only appear in the press when you are born and when you die.

Which certainly isn't Tinsley's game plan. She has leveraged her beauty and background to turn herself into a New York society brand. Highlights of her career to date include landing a job at Vogue, after being asked to do a fashion shoot at her dining club at Columbia University (a sort of unisex Bullingdon Club) styled by Plum Sykes; her work as a 'beauty ambassador' for Dior (they named a lipstick after her); designing handbags for Japanese label Samantha Thavasa; and appearing as herself in the cult TV show Gossip Girl, as well as being a character in Sykes' society novel The Debutante Divorcée.

In the last three years, the New York photographer Patrick McMullan has posted over 2,000 photographs of Tinsley on his website. Her hairstyle, with its blonde curls, is now known in Manhattan hair salons as simply 'the Tinsley'. And most recently she has secured financial backing from a wealthy Virginia businessman to build a fashion company under her own label, 'Tinsley'. Whether London will be so easily charmed remains to be seen.

'If I'd known there was going to be so much interest, I think I might have dressed differently for the polo,' said Tinsley over lunch at Aspinalls. 'I didn't realise that Cartier Polo was such a big deal and that the Queen comes to watch. Anyhow, I'm loving being in England these days. I used to come as a teenager because my mum is an interior designer.'

Tinsley Randolph Mercer comes from Virginia: her mother, Dale, is descended from Thomas Jefferson, and her father, George, is related to the 4th president of the United States, James Madison (she also has a younger sister, Dabney), although much of the family money has been made more recently from property and rug manufacturing. As a young teenager, Tinsley spent a year training at the Nick Bollettieri Tennis Academy in Florida and became a ranked national player before deciding she was more interested in art history and fashion than tennis.

But her year of training helped give her the drive that makes her stand out from the 'celebutante' pack who rule New York. 'I have always been an over-achiever,' she admits. 'I led my debutante ball. My tennis playing set me apart and made me competitive. I really want to be the best at whatever it is I do. When I grew up it was ice skating, tennis, ballet and horseback riding, and now it is building a business career and my own company around my fashion line, products and style.'

Tinsley met Topper in 1992 when he was in his second year at Lawrenceville, a co-ed boarding school close to Princeton. He was her first boyfriend and they got married straight out of university. Their first marriage ended up being annulled - Britney Spears-style - when both sets of parents objected, but the couple went on to remarry in Richmond, Virginia, in 2002.