The Downton Abbey beauty showed off her svelte figure in the stunning strapless number, which accentuated her tiny waist with its corset-style top and full skirt.
The gorgeous gown was topped off with a huge statement black bow on the back, which Lily showed off to perfection as she twirled for the hundreds of cameras.
Harvey had a breakout season in 2013, being selected to play in his first Major League Baseball All-Star Game.
She said: ‘There were times when it really got me down because I was the lead character of this film all about kindness and goodness, and all that people seemed to talk about was my waist.
‘I think it’s frustrating, the unreal aesthetics we aspire to.
They’re perpetuated by what we see and the image we give out.
The Downton Abbey star looked effortlessly stunning in a simple logo T-shirt from Spiritual Gangster and pleated midi skirt as she later turned her attention back to their company.
You don't have to be a yoga bunny to feel the love.