But while most eyes were on Kate at the glitzy star-studded affair, we couldn't help but notice how fab Sienna's blonde tresses were!
The Factory Girl actress looked a far cry from her pixie crop days as she flaunted a sleek, smooth and mega-long golden mane.
To go with her slick locks, Sienna wore a stunning beaded minidress - which was from Kate's new range (obvs) - teamed with towering peep-toe booties.
Receiving A-list treatment ahead of the event, Sienna had her locks tended to by acclaimed session stylist Luke Hersheson, who also beautified Cara Delevingne and Suki Waterhouse before the do.
Luke first ensured Sienna's barnet was super-clean by giving it a double shampoo using Kérastase Cristalliste Bain Cristal Cheveux Shampoo, £12, before applying Kérastase Mousse Bouffante, £14.62.
He then rough-dried her bonce until it was around 80% dry. To finish Sienna's blow-dry, Luke used Hershesons Ceramic Ionic Brush #2, £30, to help build volume at the roots and add shine.
Once her tresses were dry, Luke ran over pieces of Sienna's hair using Hershesons Titanium Ionic Professional Straighteners, £98 (for a more purse-friendly option, try BaByliss Pro 215 Ceramic 2039BU Hair Straighteners, £14.99), before spritzing with a mist of Kérastase Spray à Porter Tousled Effect Spray, £17.25.
Finally, he softly backcombed her hair for a modern, dishevelled finish.
What do you think of Sienna's look? Let us know by commenting below as the best comment across the site each week wins £50!
Got an iPad or iPhone? Reveal Mag is specially designed to fit!