A post on the "what's on" section of the venue's website came on Wednesday, the day before it is believed the official announcement was due to be made.
"Over the previous year, Her Royal Highness has held meetings and conducted private visits with each of these organisations", the Palace said.
Meghan will continue in her role as joint-patron for The Royal Foundation, which was set up by Princes William and Harry.
The organizations were naturally delighted to have Meghan's endorsement.
Manchester United face Arsenal at the Emirates in FA Cup fourth round
The holders pushed for double figures at the end, but had to settle for just the nine goals to take into the second leg. Pep Guardiola always speaks fondly of this competition as it was the first trophy he won since coming to England.
The Duchess of Sussex will today visit one of the charities, Smart Works, which helps long term unemployed and vulnerable women regain the skills, confidence and tools to succeed at job interviews, return to employment and transform their lives.
The Duchess of Sussex's decision to become the royal patron links to her belief in using the arts to bring people from different backgrounds and communities together.
The former Suits star then helped the ladies in finding outfits for women attending their second dressing, and she also sat in on interview coaching and preparation.
Rumours of a royal rift between the women began swirling after news emerged that Meghan and Harry will move away from Kensington Palace to make their home 25 miles away from the Duke and Duchess of Cambridge in Windsor.
Mayhew is an animal welfare charity which helps cats and dogs through community initiatives and veterinary care.
Royal patronages are not necessarily for life and the younger royals usually have fewer patronages than older royals, as they steadily acquire more. Andrews writes that the duo consider the Cotswold pad as "more of a home" than their current residence, Nottingham Cottage in Kensington Palace.
Meghan Markle has officially been announced as patron of four organisations, taking over two of those from The Queen.