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.
According to Us Weekly, Markle wore a Club Monaco multi-color shirtdress with a Philip Treacy hat, Aquazzura shoes, a Kayu clutch and a knotted belt to complete the look. Since Harry was part of the wedding party as an usher, Markle got to make some new friends among the other wedding guests.
The guests also included Princess Eugenie and fiance Jack Brooksbank, who are getting married themselves on Oct. 12.
Eugenie was recently in the headlines for getting in trouble by sharing a photo of her father, Prince Andrew, from a restricted area of Buckingham Palace on Instagram. Eugenie is the only British royal with a personal Instagram account.
The Royal Family also celebrated Markle's birthday with messages on Twitter. "Wishing HRH The Duchess of Sussex a very Happy Birthday," Prince Charles' team tweeted on the Clarence House Twitter page.
Wishing HRH The Duchess of Sussex a very Happy Birthday 🎂 pic.twitter.com/mjvSETqaqF— Clarence House (@ClarenceHouse) August 4, 2018
"Thank you everyone for your lovely messages on The Duchess of Sussex's birthday," Kensington Palace added.
Van Straubenzee, 30, is head of U.K. Financial Intermediaries for the investment management firm Brown Advisory. According to PEOPLE, he has been friends with Harry since they met at Ludgrove Prep School. Van Straubenzee also attended Harry and Markle's wedding.
Van Straubenzee's older brother, Thomas Van Straubenzee, is also friends with Prince William and is one of Princess Charlotte's godparents.
Van Straubenzee also reportedly gave a speech at Harry and Markle's exclusive evening wedding reception at Frogmore House. He reportedly joked about Harry playing "dormitory monitor" during their school days.
"All I wanted was a cuddle from my mummy," he said. "Instead, Harry nicked my teddy bear and chucked it out onto the roof."
This was the second wedding Markle and Harry attended as a married couple. In June, Celia McCorquodale, the daughter of Princess Diana's sister Lady Sara McCorquodale, married George Woodhouse at St. Andrew and St. Mary's Church in Stoke Rochford Lincolnshire. Celia wore the Spencer Tiara, the same tiara Diana wore when she married Harry's father, Prince Charles, in 1981.
The Duke and Duchess of Essex visited Dublin last month for a couple of days, and are planning their first major international tour as a married couple for this fall. They plan to visit Australia, New Zealand, Fiji and the Kingdom of Tongo, around the time of the Invictus Games in Sydney in October.
Photo credit: Samir Hussein/Samir Hussein/WireImage/Getty Images