Doria Ragland, who lives in Meghan’s native California, made a surprise appearance as she joined the duchess and Prince Harry at the garden function on Thursday.
The mother and daughter have not been seen together in public since the royal wedding on 19 May.
However, there are reports Meghan has made a secret trip to Los Angeles to see her mother since then and is considering moving to the UK to be close to her only daughter.
They looked happy and relaxed together as the duchess was reunited with women from a Grenfell community group who have been cooking together following the fatal fire last June.
Ms Ragland was heard introducing herself to guests by saying: “Hi, I’m Meg’s mom.”
Her appearance alongside her famous daughter was unusual as non-royal family members do not attend official royal engagements.
Meghan’s husband and mother looked on proudly as she gave a speech to launch the book, telling them the multi-cultural kitchen has been integral in helping her feel welcome in London after moving from Canada where she lived while filming the popular TV series Suits.
She said the project had been a “tremendous labour of love”.
The duchess was wearing a striking petrol blue long coat by one of her favourite Canadian brands, Smythe – which sold out within minutes – with a pleated midi skirt by her friend Misha Nonoo, a high neck bodysuit by Tuxe and black suede Sarah Flint heels.
Earlier this week Kensington Palace released a video of Meghan cooking with the women in support of the new recipe book – called Together: Our Community Cookbook – which aims to raise money to support the local kitchen.
Meghan came up with the idea for the book during her first of many visits to the Hubb Community Kitchen in North Kensington, just a couple of miles away from her new palace home.
As part of her first solo project since becoming a duchess, she thought the book could help fund the kitchen opening more than just two days a week.
Guests at the reception inside a tent in Kensington Palace’s gardens enjoyed dishes made by the women, many of which have been handed down through generations.
There was a large range on the menu, including coconut chicken curry, aubergine masala and a range of chapatis and sharing dips.
Caramalised plum upside-down cake and spiced mint tea was also served.
Meghan joined the women as they cooked the dishes and helped with preparing the feast, making chapatis and turning koftas on the grill, before they all sat down with Prince Harry, Ms Ragland and their guests.
Guests included members of the local community, staff from Ebury Press, which is publishing the book, members of the Al Manaar Muslim Cultural Heritage Centre, where the kitchen is based, and staff from the Royal Foundation.