Markle arrived at the Commonwealth Day Service with Harry, his elder brother Prince William, the second in line to the throne, and his wife Kate, Duchess of Cambridge.
The service is Britain's largest annual inter-faith gathering and during the event Markle joined members of the royal family in greeting attendees.
Since announcing their engagement in November, Markle and Harry have made regular public appearances together, but Monday's service was the first official outing with the queen.
The couple will marry at St George's Chapel in Windsor Castle on May 19.
In preparation for the ceremony Markle was reportedly baptised last week by the Archbishop of Canterbury, Justin Welby, leader of the Church of England.