They were joined by Prince Harry, Kate Middleton, Prince William, Prince Charles and more.
The 36-year-old humanitarian, who has been busy gearing up for her May 19 wedding to Prince Harry, attended her first royal public event alongside Her Majesty on Monday at the Commonwealth Service at Westminster Abbey.
Meghan wowed in white for the occasion, donning a cream-colored Amanda Wakeley coat with a white Stephen Jones hat and navy heels, seemingly taking a style cue from her future sister-in-law, Kate Middleton. The brunette beauty completed the look with minimal makeup, leaving her tresses down with loose curls as she walked side by side with her fiance.
Queen Elizabeth looked equally fab in an all-burgundy ensemble, consisting of a jacket and hat paired with her signature black shoes, black gloves and a matching handbag.
Having Meghan at the event was a break in royal protocol for the Queen, but it's not the first time she has made an exception for the former Suits star to join them prior to her nuptials to Harry. The bride-to-be was previously invited to spend Christmas with the family, and even attended their Christmas Day service.
The Commonwealth Service was a true family affair for the royal family on Monday. In addition to the Queen, almost all of the core members of the Windsor family, including Prince Harry, Prince William, Kate Middleton, Prince Charles, Camilla Parker Bowles, Sophie, the Countess of Wessex and more, were in attendance.
The Commonwealth Service is the largest inter-faith gathering in the United Kingdom and is currently being televised on the BBC. The theme of this year's service is "Towards a Common Future," which "emphasizes how the Commonwealth can address global challenges and work to create a better future for all citizens through sub-themes of sustainability, safety, prosperity and fairness," the Palace previously said in a statement.
Former One Direction heartthrob Liam Payne is set to perform a cover of John Mayer's "Waiting on the World to Change" at the service, and there will be politicians and dignitaries from all over the world in attendance. Following the service, William, Kate, Harry and Meghan will walk through Dean's Yard to attend a reception.
The Queen became head of the Commonwealth at the age of 26. Since then, it has grown from seven nations to 53 members, representing 2.4 billion citizens.
Next month, Commonwealth will be kicking off it's Commonwealth Games on Australia's Gold Coast, beginning April 4. One year ago this week, Queen Elizabeth kicked off the games outside of Buckingham Palace by passing a baton to start a 12-month journey around the Commonwealth ahead of the games' start.
These events come ahead of two extra special dates in the Royal family -- the birth of William and Kate's third child in April and the wedding of Meghan and Harry on May 19.
For more from the Royals, watch the clip below!