In the penultimate room of Sargent and Fashion at Tate Britain, there’s a small painting that, to me, encapsulates the entire show. Two Girls in White Dresses (c.1911) depicts a couple of figures silhouetted against a grassy meadow in the Italian Alps. Both are modelled on the artist’s niece, Rose-Marie Ormond, but the main character here is the clothes. Three-quarters of the canvas are taken up by voluminous stiff and creamy-white skirts and patterned cashmere shawls, laid on in thick liquid strokes.

Travelling to London from the Museum of Fine Arts in Boston, this dazzling exhibition celebrates the great portrait painter John Singer Sargent’s ability not only to paint his sitters, but to style them too. Spread across nine thematic rooms are 60 works made mostly in London and Paris at the turn of the 20th century, several shown alongside the garments and accessories pictured. Opening hot on the heels of London Fashion Week, the show offers an alternative runway that invites us to reconsider the American expatriate’s work according to attire. The models include his key patrons – typically, but not always, wealthy members of high society – as well as fellow figures in the arts, politicians and performers. And the fashion? Well, it speaks for itself.

Back to the beginning and we’re greeted by Lady Sassoon wrapped in a black taffeta opera cloak that’s displayed in a glass case nearby. Sargent’s role as stylist is evident in the way he folded the cloak back to reveal the lush pink lining – barely visible in real life – and pinned to it matching pink roses. The outfit is complete with an astonishing hat of ostrich feathers that shows off the sitter’s porcelain skin, a string of pearls and bangles on both wrists. Lady Sassoon was a lover of music and the visual arts, and the effect, as she turns towards us, is elegant and sultry.