Prince Charles offered similar advice to his son Harry.
However, Meghan later appeared to claim she was not supported by the Windsors, famously telling an ITV documentary: "Not many people have asked if I'm OK."
A friend of Markle's shared: "Meg was really grateful to Camilla, who was very supportive and invited her out for private lunches, particularly around the time of her marriage.
"She listened to her and understood that it's really difficult joining the royal family from an otherwise 'normal' life."
Camilla was hounded with harsh press and hostility due to her relationship with Charles when he was still married to Diana.
"She was very sensitive to Meghan and provided her with support, advising her to ride out the storm and that it would all pass – but ultimately Meghan didn't listen," the friend added.
Camilla was blamed for the breakdown of Charles's marriage and, after Princess Diana's death in 1997, received widespread criticism.
It is understood that when the Duchess of Cambridge married Prince William in 2013, Camilla also took Kate out to lunch to offer similar help and advice.
A palace aide said: "The Duchess of Cornwall does understand what it's like to come from a normal life and then land in the completely extraordinary world that the royal family exist in.
"As a consequence, she tries very hard to make people welcome and help them find their feet, and has done so for many years.
"Her consciously offering to mentor and guide is something that she really does try and do. That applies to many people, not just the Duchess of Cambridge and the Duchess of Sussex."