The cake will incorporate "bright flavours of spring" and will be covered in buttercream and fresh flowers, according to Kensington Palace.
Meghan and Harry haven chosen Claire Ptak, who is from California, to make the creation for their special day.
Ms Ptak started out on a stall in Broadway Market and is now going to design what is sure to be one of the most famous Royal wedding cakes in history.
The pastry chef opened Violet Cakes in Hackney in 2010, which specialises in using seasonal and organic ingredients in their cakes.
WHO IS MEGHAN AND HARRY'S WEDDING CAKE DESIGNER? CLAIRE PTAKKENSINGTON PALACE
For their wedding cake Prince Harry and Ms. Meghan Markle have chosen pastry chef Claire Ptak
Ms Ptak was previously interviewed by Meghan, 36, for her lifestyle wbesite, The Tig.
Kensington Palace said Harry and Ms Markle "are very much looking forward to sharing this cake with their wedding guests".
They tweeted: "For their wedding cake Prince Harry and Ms. Meghan Markle have chosen pastry chef Claire Ptak, owner of the London-based bakery Violet Cakes.
"Prince Harry and Ms. Markle have asked Claire to create a lemon elderflower cake that will incorporate the bright flavours of spring. It will be covered with buttercream and decorated with fresh flowers.
"Claire Ptak, who was raised in California, focuses on using seasonal and organic ingredients in her cakes. Ms. Markle previously interviewed Chef Ptak for her former lifestyle website The Tig.
"Prince Harry and Ms. Markle are looking forward to sharing the cake with guests at their wedding at Windsor Castle on May 19th."
California-raised Ms Ptak formerly worked as a pastry chef for chef Alice Waters at Chez Panisse in Berkeley, California.
She started her own business as a stall in Broadway Market, east London, cooking from home, before opening her organic bakery.