The 34-year-old mother of two wore a floor-length white Self Portrait dress, which featured a daring thigh-high slit. She added a dash of color to her regal ensemble with matching maroon-colored accessories -- a dainty poppy brooch, velvet clutch and pointed toe stilettos.
Keeping the focus on her gorgeous gown, the brunette beauty opted for minimal makeup and bouncy, brushed-out waves.
Middleton made an appearance at the premiere because the film benefits one of her charities, Action on Addiction, an organization that helps those struggling with addiction. Inside the private screening, she got to meet some of the stars of the movie, including Bob the street cat.