The former Suits actress wowed in a shimmering brocade dress and a £900 white coat by Amanda Wakeley as she and Prince Harry chatted to guests at the reception.
The event is being hosted by the Queen and is taking place at Buckingham Palace.
Charles arrived at the event with Camilla and his sons and their wives - William and Kate and and Harry and Meghan in step with the heir to the throne as he entered the White Drawing room.
Meghan, who is pregnant with her first child cradled her bump and smiled as she chatted to other guests.
She carried her £205 oyster silk clutch by Wilbur and Gussie and wore her Paul Andrew 'Pump it Up' stilettos which retail at £475.
Her £900 white 'Crombie' style coat is by designer Amanda Wakeley who has long been a royal favourite.
She accessorised with Les Plaisirs de Birks Bar Earrings, which cost £213.45.
Kate Middleton wore a button up high neck mint dress with a balloon sleeve.
She mingled with Stereophonics frontman Kelly Jones before going to view the display with the other senior royals.
This marks the first time Meghan and Kate have been pictured together at the same event since attending church together on Christmas Day at Sandringham.
Rumours of a feud between the Duchesses started swirling shortly after the Sussexes announced they were moving from Kensington Palace to Frogmore Cottage on the grounds of Windsor Castle.