The Oscar winner looked stunning in a black sunhat and Alexander McQueen black-and-white dress as she enjoyed the event with her brother (and Ellen's Oscar selfie crasher) Peter Nyong'o.
"Polo dates. @nyongolaflame @sahlovestravels #VeuveClicquotPoloClassic," the 12 Years a Slave star captioned a shot of herself, Peter and a pal riding in a car on the way to the event.
Meanwhile, Dakota Johnson was also on hand for the event. While she stayed mum on detalis about her upcoming flick Fifty Shades of Grey, the actress did give us the scoop on what she thinks the sexiest thing is that a woman can wear.
"I think the sexiest thing is a hat. It's very sexy!," she told E! News exclusively. And it just so happened that she wearing just that, along with a black and white sundress and matching heels.
Before heading out for the day, the actress also decided to play a bit of a prank while she was there, running off with a loaf of bread…and then posing for pictures with it.
-Additional reporting by Jennifer Cooper