Dressed in a bright coral outfit with a matching hat, the 87-year-old reigning monarch was all smiles as she was accompanied by her husband Prince Philip.
The royal couple were greeted warmly at the entrance by the Dean of Westminster John Hall and Sub-Dean and Archdeacon Jane Hedges. Before the start of the service, the Queen broke the flag in a traditional ritual.
The Queen is all smiles as she arrives for the service
First-in-line to the throne Prince Charles and his wife Camilla, Duchess of Cornwall, were also spotted arriving for the service. The Duchess looked elegant in a cream dress and coat and accessorised with a three-strand pearl necklace and feathered fascinator.
Following shortly behind was the Queen's youngest son and birthday boy Prince Edward, who was celebrating his milestone 50th birthday. The Earl of Wessex was accompanied by his beautiful wife Sophie, Countess of Wessex, who coordinated with Camilla in a cream coat and pale pink heels.
Where were Prince William and Kate Middleton? Find out here.
As head of the Commonwealth, the Queen is expected to take centre stage in the service, delivering a speech that will later be broadcast to all of the Commonwealth Nations.
Camilla and Prince Charles
Her Majesty had already released a pre-recorded message before the service, in which she praised all 53 nations that make up the Commonwealth.
She spoke of the united community as "a powerful influence of good for the future" and commented on how much they had achieved by working together.
"Experiences of life differ widely throughout the Commonwealth, and we each make contributions from sometimes very different viewpoints," she said, as she focused her speech on this year's theme, Team Commonwealth.
"But we are committed to the same goals. Together we offer each other encouragement and draw strength from this mutual support. The understanding that we belong together, and are able, through teamwork, to achieve far more than we could do alone, has always been at the heart of our approach."
Check out the Queen's special gift to baby Prince George — a personalised corgi t-shirt.
Sophie and Prince Edward
During the service, the royals will enjoy music, hymns and readings and an address by schoolgirl campaigner Malala Yousafzai. Former sporting stars Lord Sebastian Coe and Baroness Grey-Thompson will both give a "reflection".
A performance by acclaimed Mobo award winner and Brit nominee Laura Mvula has also been scheduled. A poem — which was especially commissioned for the event — will also be read by South African writer artist Phillippa Yaa de Villiers.
The annual Commonwealth Day service takes place on the second Monday of March.
Watch Kate Middleton and the Queen host a star-studded RADA reception below: