The 36-year-old actress completed the elegant ensemble with nude heels and a black clutch, also from the storied French fashion house, as well as a sheer black fascinator by Stephen Jones and a low bun.
Commenting on Meghan's dress, celebrity stylist and designer Lucas Armitage (the-timeless.com) told MailOnline that Meghan had made a "bold choice" by choosing black. "I remember the Battle of Britain being fought in the skies above us and we shall never forget the courage and sacrifice of that time", she added, referring to the decisive encounter between the air forces of Britain and Nazi Germany over southern England in 1940.
Dressed in a suitably green Givenchy dress, Meghan and Prince Harry were briefly greeted before taking off for their first engagement.
The royal brothers wore their military uniforms as they arrived at Westminster Abbey Tuesday morning for a service marking the centenary.
Kate and Meghan stunned at the RAF centenary.
These include Operation Ruman - the UK's humanitarian assistance operation in the aftermath of hurricane Irma which hit the Caribbean in September 2017 - and the RAF's Quick Reaction Alert Typhoon Aircraft, providing a United Kingdom air defence capability 24 hours a day, 365 days a year. According to the outlet, the royal family began the London festivities with a service at Westminster Abbey followed by a parade "of up to 1,300 serving personnel" and a commemorative flypast over the palace.
Prince William served a stint in the RAF Search and Rescue service in North Wales, and Prince Harry also briefly took part at RAF Shawbury. Meetings with The President of Ireland, Michael Higgins, and with The Taoiseach, Leo Varadkar, are both planned.