Samantha and her partner were then photographed handing a note to the guard before they left the premises and were spotted buying Harry and Meghan masks nearby.
In the interview with Good Morning Britain, the 74-year-old leaked details of private conversations with Meghan and Harry before telling United States press he felt "frozen out".
Meghan Markle isn't phased by her half-sister's recent unexpected visit.
Likewise, a source tells ET that on Oct 1st Samantha and boyfriend Mark Phillips were seen outside Kensington Palace.
But her attempts to meet the Duchess of Sussex face-to-face were stalled when a security guard refusing to let her pass.
But, on Sunday, Samantha teased her Twitter followers, writing, "Everybody would love to know what's in that letter".
The source told the Mirror: "This is the closest Meghan has come physically to that side of her family since she married into the royal family but palace staff had clearly been instructed not to allow her access".
The public spat was accentuated after Samantha did not receive an invite to the royal couple's May wedding. Grant, who has multiple sclerosis, was in a wheelchair pushed by her friend. However her arrival at palace gates has got her no further in her quest to speak face-to-face with Meghan it would seem. "I can't give the details of that but I really wanted the world and really my sister and the British Royal family to know what happened and know the truth about a lot of things". "She's mindful that she's representing the Queen and the royal family and doesn't want to put a foot wrong".