British models and socialite sisters Cara and Poppy Delevingne and Peruvian fashion photographer Mario Testino also joined the the front row at Paris Fashion Week's most star-studded event yet.
The ex-Beatle rushed in at the last minute to take his place, setting off an explosion of camera flashes and taxing the ushers who had to shoo the paparazzi away so the catwalk parade could start.
Under the ornate, gilded ceiling and elegant chandeliers of Paris's 19th century Opera building, Stella McCartney unveiled a confident autumn-winter collection.
She married simple, relaxed lines to silhouettes exaggerated by overly long sleeves, hems and pant legs.
Black dresses hugging shoulders and waists were joined by many grey moulded woollen outfits ranging from suits to deceptively simple dresses.
(Stella McCartney acknowledges the applause at her Paris fashion show. Photo: AFP)
Convincing faux fur
In her production notes, the 43-year-old said her themes were "celebrating freedom" and "teasing sensuality".
Life-long vegetarian and animal rights advocate Stella also presented long-haired coats in black and white, made from incredibly convincing faux fur.
Eveningwear employed canny touches of other materials along shoulders and ribs to make lines appear even slimmer.
A storm of applause broke out at the end of the show, as Paul McCartney stood, hugged Harrelson and West, and moved with his American wife Nancy Shevell through the crowd to congratulate his daughter backstage.
Stella McCartney started her own brand in 2001, and has since built a rock-solid name of her own, far from the shadow of her father's fame.
Her shows are now top events at the yearly Paris Fashion Week, up there with Kenzo, Dries Van Noten and Balmain.
Her 72-year-old father, who composed all-time classics "Let it be" and "Yesterday", is still performing.
Last week he announced he was extending a world tour with nine dates in Europe, including an emotional return to the Beatles' native Liverpool.
Kanye West, the rapper husband to US reality TV star Kim Kardashian, teamed up with McCartney for a single released in January called "Only One".
The collaboration, which has West singing a soft ballad while McCartney plays the organ, was released after the two met early 2014 in Los Angeles.
It was said to be inspired by West's daughter with Kardashian, North.