Their appearance was for a good cause, as they raised money for the Paddle and Party for Pink fundraiser in the Hamptons, but 28-year-old Mary-Kate made sure to keep her engagement ring tucked away between herself and her older beau as they cuddled up together at the party.
CLICK ON PICTURES FOR GALLERY
The ring, which was first spotted during Paris Fashion Week, is an intricate Cartier piece dating from 1953 — a fitting choice considering the blonde beauty's penchant for vintage clothing. It features a four-carat central diamond flanked by 16 sapphires, and the "petals" are set with single-cut diamonds.
The fashion designer bypassed the pink dress code, opting instead for an all-black ensemble. Her twin sister, Ashley, was also at the event, which kicked off with a paddle boarding race, followed by a cocktail party which lasted well into the evening.
A host of big names posed for photos with the paddleboards which were later auctioned off for the charity.
Mary-Kate, and Olivier, 45, have been together for over two years, and live in the lower east side of Manhattan.
Although she has not spoken openly about their relationship, earlier this year Mary-Kate admitted she had considered making her own wedding dress.
"I'm, like, not even there yet," she said. "I think it may be a matter of getting five or six, or making two or three and being stressed out. That seems kind of far away. We'll take one step at a time."
Younger sister Elizabeth, who is engaged to Boyd Holbrook, also recently spoke about her own wedding plans, and when asked about whether she had picked a wedding date and whether it would coincide with her big sister Mary-Kate, said, "No, I just told her she gets to pick a date first!"