Last week, she said: "It took 10 years to get to that stage and to have the confidence that [working with her father] wasn't total nepotism. It was the right time."
The BBC's online video footage showed him almost sitting on his daughter's lap in the auditorium as they looked down over the orchestra pit in rehearsals.
So why now? This month, Stella turned 40. Her fourth baby, Reiley, is nearly one. And this year marks the 10th anniversary of her label, which she founded after leaving design house Chloe. Maybe she finally feels that she can do whatever she wants and not care what anyone else thinks.
For years, her career has been dogged by an off-the-cuff remark made by Karl Lagerfeld. When she was appointed in 1997, not long after her graduation show at Central Saint Martins in London, the Chanel maestro went on the record to denounce her: "Chloe should have taken a big name. They did, but in music, not in fashion. Let's hope she's as gifted as her father."
She has had to work hard to shake off this accusation. Her association with a coterie of celebrity mates hasn't helped: Madonna, Gwyneth Paltrow, Kate Moss (whose wedding she recently attended: "It was everything you would think it would be."). Naomi Campbell and Yasmin Le Bon modelled her graduation collection in 1995 for free and the collection was shown with a song written by her father booming out.
Despite Vogue's review of her first collection - "[it] quickly dispelled any doubts about her talent" - during her time at Chloe she was still very much associated with her rock star roots. She designed Madonna's wedding dress in 2000 and when she received the VH1/Vogue designer of the year award the same year, it was presented to her by her father. It has been claimed that when she left to set up her own label in 2001, she argued for it to be called simply Stella, but Gucci was not keen for her to lose the McCartney cachet.
In interviews she has repeatedly referred to her father in inverted commas. To the BBC: "You have that emotion of feeling very proud and then you realise that you are working with 'Sir Paul McCartney'." To the Daily Beast: "He is Paul McCartney! How awesome is that?" Isn't it strange to refer to your own father like this? Perhaps it demonstrates a certain media savvy. That she knows what everyone else is thinking and voices it. But it also shows a certain distance and awe. She has had an acute awareness of what her family name means to outsiders. Her father's band Wings were on tour for the first nine years of her life; she travelled the world with them from the moment she was born.
After years on the road, she grew up on an organic farm in Sussex, southern England, with siblings Mary, James and Heather, Linda McCartney's daughter from her first marriage. The focus was on an "ordinary life" (or ordinary for celebrity vegetarians with a pseudo-socialist conscience anyway): riding horses, raising sheep and attending the local state school.
She has acknowledged that her privileged upbringing gives her the confidence to do what she wants. "The greatest luxury of having the parents I had was that it has enabled me not to have to compromise. In the back of my mind, I always knew - if this all goes horribly wrong, I'll be all right. That's an option that most people just don't have, financially."
Her family background has given her an enviably comfortable sense of self. In person, she comes across as relaxed, down to earth and at ease - as I found when I interviewed her a couple of years ago. In fashion circles, she is seen as stylish but unpretentious. She rarely wears makeup and is seen as having made a genuine effort to use models who aren't size zero, a trend which she has described as "a joke". Her own line is described as being cut "for real women".
Married to Alasdhair Willis, ex-publisher of Wallpaper* magazine, they have four children under the age of six, sons Miller and Beckett, and daughters Bailey and Reiley. The first three were born within three years.
But she is not prone to declarations about her life as a superwoman.
Even the "mummy diary" she submitted to Gwyneth Paltrow's newsletter GOOP earlier this year was not quite as nauseating as it perhaps might have been: "I try my best to juggle the three monkeys and keep them all in one piece till bed time."
She appears to have a sense of humour about her collaboration with her father, asking him if he "really wanted tutus": "It doesn't take a rocket scientist to figure out that you have to use stretch materials if you're working with ballerinas. I'm pretty up to date on Dad's music. I've seen it all my life and how he works ... so that part was a no-brainer for me. My role was to design the costumes and tell the storyline."
Despite the fact it was a New York event, there were some suggestions that this collaboration gave a sheen to the close of London Fashion Week. But although she designs for Adidas in London and is based there, Stella McCartney shows her own label in Paris, where her spring catwalk collection debuted last week to major acclaim.
This unusual move towards working with her father suggests Stella is more open to change - and to being linked to her background - than in previous years. Her name is at the top of a wishlist of British designers being desperately courted for a fanfare return to London for the Olympics in 2012. Respected d in her own right on the international stage, maybe now she's confident enough to make a headline-grabbing return to London Fashion Week. She is already designing the Adidas sportswear for the British Olympic and Paralympic teams next year.
Meanwhile, Ocean's Kingdom certainly isn't about Stella getting her groove back. Because she never lost it. But it is about her giving her dad some help to find his. His attempts at exploring musical genres have been ridiculed for decades. (Who can forget The Frog Chorus?) The critics have damned Ocean's Kingdom with faint praise: "Certainly not an embarrassment"; "Never less than agreeable"; "Ballet for beginners".
Rolling Stone's kinder verdict? "An artist risk that seems to be paying off."
The costumes, however, are an unmitigated triumph. Almost as if now the Beatle needs his daughter more than she needs him.
The McCartney file
Born: September 13 1971 in Lambeth, London. Third in a family of four. Married since 2003 to creative consultant Alasdhair Willis, 40. Four children aged between 6 and 1.
Best of times: Her first collection for Chloe in March 1997 was hailed as a triumph by Vogue. In 2001, she was recruited by Gucci to develop her own label.
Worst of times: The first Stella McCartney show in autumn 2001 was described as "a car crash". It featured a T-shirt with the word BRISTOLS emblazoned across it. In November 2007, she designed a silver necklace featuring a single leg in an apparent attack on her father's estranged wife, Heather Mills.
What she says: "The way my parents brought me up to see the world is still absolutely key to what I am about."
"When I get up in the morning, number one I'm a mother and a wife and number two I design clothes."
"If I look like I'm putting my hand up and saying, look at me, I'm above reproach, then I'm going to get screwed. That's the nature of the country we live in."
What others say: "Her family background is completely irrelevant. She cares about her career. She works very seriously." - Domenico De Sole, former CEO of Gucci, now chairman of Tom Ford.
- OBSERVER
