He was referring to the Duchess' candid confessions in the recent ITV documentary Harry & Meghan: An African Journey.
The 54-year-old journalist said Meghan should be more like the Queen, who understands that you never explain, never complain and do your duty and everyone loves you.
"The moment you start yapping away moaning about everything, people go, ‘Hang on, you’re living at our expensive palaces, with servants.’ So maybe get a better sense of reality,” he said.
In the documentary, Meghan, who only gave birth to baby Archie five months ago, revealed that the pressures of being a mum, a newlywed and a royal was "a lot" and taking its toll.
WATCH: Meghan Markle says she is 'not okay'
Piers, who claims to have once been close to the Duchess, went on to say Meghan was focused on becoming a royal and once she met Harry she "dropped" a number of her friends, including himself.
He said now that Meghan has met and married her prince charming, she should understand that there comes a sense of duty and responsibility with the role.
"if you’re going to do it, I think take the advice of his grandmother. Don’t complain and don’t explain, just do your duty. Do your job," he said.
Pier's comments came after one of Meghan's friends made several revelations about the Duchess in the wake of her emotional words in the ITV documentary.
Daniel Martin, who is Meghan's longtime makeup artist believes the documentary was "helpful" for the royal, telling CBS This Morning on Thursday:"She has a lot on her plate and I feel like she just wants to make sure that her boys are safe."