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.
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.
The US actress' attendance at the Westminster Abbey service marks a major milestone for Harry's bride-to-be, as it the first time she has taken part in an official event with the Queen.
The Commonwealth Service was a true family affair for the royal family on Monday.
The service, which is the largest annual inter-faith gathering in the United Kingdom, is held every year to celebrate the Queen's lifetime commitment to the Commonwealth as Head of its 53 countries.
Her father, Thomas Markle, is also said to be unhappy with Prince Harry's comments about the royals being like the "family she never had".
London will soon host a gathering of leaders from the family of nations and the Queen, who is head of the Commonwealth, also paid tribute to the biennial event as an example of how consensus can "help to create a future that is fairer, more secure, more prosperous and sustainable".
Former One Direction band member Liam Payne will perform during the service - but the Duke of Edinburgh, who has effectively retired from public duties, will not be attending.
The prince said they looked forward to travelling around the Commonwealth, meeting some of the 2.4 billion people from the 53 member states.
The source added: "Some of the royals aren't happy about the idea, so they suggested that William do it".
Next month, Commonwealth will be kicking off it's Commonwealth Games on Australia's Gold Coast, beginning April 4.
Meghan Markle and Prince Harry recently greeted crowds during their trip to Birmingham, and Meghan was spotted wearing a diamond cross bracelet - which is believed to represent her recent confirmation.
For more from the Royals, watch the clip below!