The two couples attended the inaugural Royal Foundation forum in London, where they discussed their ongoing charity work. Prince William, Prince Harry and Middleton founded the organization and Markle will become a patron after her marriage to Prince Harry, according to People.
Middleton, who is expecting her third child with Prince William, wore a long-sleeve blue dress, with Markle in a complementing navy wrap dress. Prince William said he was "particularly happy" to have Markle at the event.
"Ten years ago Harry and I were still serving full-time in the military, but we were starting to look to the next stages of our lives," the prince said while explaining how he and his brother started the foundation.
"The task for us would not be to reinvent the wheel. Instead, our job was to follow the example of those who had come before us, hold onto the values that have always guided our family, but seek to engage in public life in a way that was updated and relevant for our generation," he added.
The Royal Foundation supports a number of campaigns, including Heads Together, which strives to break the stigma about mental health issues. Kensington Palace shared a photo on Twitter of Prince William, Middleton, Prince Harry and Markle at the forum.
"THR & Ms. Markle spoke to @TinaDaheley about the inspiration behind The Royal Foundation, the issues that are close to their hearts, and their ambitions for the future," the palace wrote.
Prince William and Middleton married in April 2011, and share 4-year-old son Prince George and 2-year-old daughter Princess Charlotte. Prince Harry and Markle got engaged in November, and will marry May 19 at Windsor Castle.