Taking to his Twitter account, the 59-year-old shared a photo of the brand new family flag which protrudes from the roof of the main building on the lavish estate.
"New Spencer flag flying above @AlthorpHouse The old one was getting a bit weather-beaten," the author penned.
In the clip, the flag was furiously waving in the autumn wind against a picture-perfect blue sky. Royal fans flocked to the comments section.
"I remember being on the bus and the guide saying the flag would signify that you were home. And that day you were but perhaps you are always there during the visiting season. I still think about the visit and Diana often," one fan penned.
A second added: "Beautiful day for hanging it!!". Meanwhile, a third penned: "Stunning in the sunlight and clear blue skies."
The post came just hours after Charles shared a special message to Prince Harry and Meghan Markle on Thanksgiving.
"Happy Thanksgiving to all Americans - from Manhattan to Malibu (both of which I’ve visited in the past few weeks)."
The Earl didn't confirm whether he had met up with his nephew and niece-in-law but they have always had a close relationship.
Althorp has been undergoing big changes thanks to Charles' wife Lady Karen Spencer. On Sunday she revealed the fabulous makeover the sprawling gardens on the ground have undergone.
The Countess captioned the image: "Wow, drying garden has been cleared!" The space was enclosed by a wall and looked clear and ready for the new season.
The gardens on the estate are also home to Princess Diana's final resting place, coined an "oasis of calm" by Charles. The area is only accessible via boat and is off-limits to the public.
In 1998, Diana's brother reportedly said the lake would "act as a buffer against the interventions of the insane and ghoulish, the thick mud presenting a further line of defenence. He went on to say: "We all agreed that, with its beauty and tranquillity, this was the place for Diana to be."