Harry appears to have given his bride a poignant wedding day gift - an impressive, emerald cut aquamarine ring which belonged to his late, beloved mother.
Meghan was spotted with the large gem on her right hand as the couple made their way to their evening do.
Diana died when Harry was just 12 after she was killed in a tragic car crash in Paris.
Meghan switched from her stunning elegant, formal pure white wedding gown by Clare Waight Keller for Givenchy, to a glamorous lily white halter neck by Stella McCartney for the black tie evening do.
The US star - now a duchess and an HRH - is giving a speech at the party, which is being hosted by her father-in-law the Prince of Wales.
Harry drove his new wife from Windsor Castle in a silver blue classic convertible Jaguar E-Type Concept Zero which was originally manufactured in 1968 and has been converted to electric power.
The happy couple waved and smiled as they headed to meet their friends at the celebration, which is expected to stretch into the early hours.
Meghan wore shoes from Aquazurra made in silky satin, with nude mesh, with soles painted in baby blue.
Her hair - in a relaxed up do - was styled for the reception by George Northwood.
The car’s number plate bore the date of the wedding - E190518.
Guests were able to rest and change before the start of the evening celebrations following the lunchtime reception in the castle’s medieval St George’s Hall hosted by the Queen.
The closest friends of the newlyweds will be among those in attendance including Canadian stylist Jessica Mulroney and other confidantes such as Benita Litt and designer Misha Nonoo.
The 17th century royal residence stands about half a mile south of Windsor Castle in Windsor Home Park.
It offers the new Duke and Duchess of Sussex the privacy to celebrate their marriage away for the cameras.
The name Frogmore is derived from the frogs which have always lived in the low lying marshy area.
It was built in around 1680 by Charles II’s architect and in 1923, the Queen’s parents - the future George VI and Queen Elizabeth - spent part of their honeymoon there.
It is only open for people to view on three ticketed charity open days a year, as well to pre-booked groups in August.