The Factory Girl star showed off her slender waistline as she tucked her shirt into the top of her khaki and black plaid trousers.
Sienna wore a pair of thick socks and black leather Gucci loafers, while carrying a large black handbag in one hand.
The mother-of-one wore her golden blonde locks in loose waves and appeared to be make-up free.
The Foxcatcher star seemed to be amused by something on her phone, giggling away as the pair looked at the screen with glee.
