The couple, who had their own wedding on May 19 at St. George's Chapel at Windsor Castle, celebrated the wedding of Harry's friend Charlie Van Straubenzee and Daisy Jenks in Surrey.
"Happy Birthday to HRH The Duchess of Sussex" the picture was captioned.
Meghan Markle's half-sister has used the opportunity of the Duchess's birthday to take a swipe at her - comparing her to Disney villain, Cruella DeVil.
Meghan paired the look with a matching Philip Treacy fascinator, black cut-out heels from Aquazzura and a belt.
Prince Charles and Camilla also posted a sweet snap to mark Meghan's first birthday as a royal.More news: Martial not a Bayern Munich target, Rummenigge confirms
What a year this has been for Meghan Markle! The Duchess was all smiles as she arrived at the church, chatting happily with fellow guests.
Prince Harry and Meghan, who have been spending their spare time at their country home in the Cotswolds, will be joining Queen Elizabeth shortly at Balmoral for the annual royal family summer visit to the Scottish Highlands.
For last year's celebrations, she spent her birthday with her husband-to-be, Harry, in Africa.
So is it to do with royal protocol, or is the former actress taking fashion pointers from her sister-in-law? "Two children would be enough for Harry", he added.
They aren't the only Royal couple thought to have nicknames for one another.
Meghan pointed to the product range and said, "I'm sure at some point we'll need the whole [lot]". "And hopefully we'll start a family in the near future".