The first-time grandmother travelled up to north Wales with the new family as Prince William returned to work as an RAF search and rescue helicopter pilot last week.
Kate's parents Michael and Carole were the first people to visit Prince George when was born on 22 July – and grandmother Carole has been close by ever since.
Carole and her eldest daughter Kate enjoy a famously close relationship so the 31-year-old no doubt appreciated her help and support in her first few weeks as a mum.
The Duke and Duchess of Cambridge took their newborn baby George to the Middleton family home two days after he was born at the Lindo Wing of St Mary's Hospital in London.
William remained at the Berkshire family home for two weeks of paternity leave before returning to Wales to resume his duties.
The 58-year-old has taken to being grandmother of the future King with "gusto", according to the Mail Online.
In a break with royal tradition, the Cambridges have chosen not to hire a nanny which means it is capable Carole who has stepped in help Kate in her new role as a mother.
It is thought that Carole went with the new family by car from Berkshire to William and Kate's Anglesey farmhouse.
After staying for a few days on the island, William dropped his mother-in-law at Bangor train station to take the train back to the south.
New dad William made an appearance at the Anglesey show this week. He took the opportunity to thank local residents for their hospitality before he and Kate prepare to leave the island when his tour of duty ends next month.
"I know that both of us will miss it terribly when my search and rescue tour of duty comes to an end next month and we have to move elsewhere," the Duke said.
"From the bottom of my heart, thank you for making my wife and me so welcome when we arrived here, as you do thousands of visitors each year.
"This island had been our first home together, and it will always be an immensely special place for us both."
The new father has been stationed as a pilot at RAF Valley at Anglesey since 2009. It is largely thought that he will return to London to join his regiment, the Household Cavalry's Blues and Royals, and care for his new family there.