The royal biographer explained that Diana would've been stunned by Meghan's public speaking skills.
"Diana was desperate to be known for speeches, rather than fashions, and took lessons to learn how to deliver talks.
"She would have been taken aback on first meeting Meghan, a force of nature able to speak with passion and precision – and all without notes.
"But as she got to know her and see the vulnerable side of a woman who suffered mental trauma during her first pregnancy – as did Diana – they would have bonded."
In his and Meghan's official interview following their engagement, Harry said that Diana would've loved his then-fiancée.
"They would be thick as thieves, without question, I think she would be over the moon, jumping up and down, you know, so excited for me, but then, as I said, would have probably been best friends with Meghan," the redheaded royal said at the time.
"So no it's, you know, it is days like today when I really miss having her around and miss being able to share the happy news. But you know with the ring and with everything else that's going on, I'm sure she's …"
"She's with us," Meghan added.
"I'm sure she's with us, yeah, you know, jumping up and down somewhere else," Harry remarked.
WATCH BELOW: Prince Harry on Princess Diana loving Meghan Markle. Post continues after video...
Princes William and Harry are set to reunite to unveil a statue of Diana on Thursday, which would've been her 60th birthday.
Neither Kate nor Meghan is set to attend, the latter will stay in America with her and Harry's children Archie and Lilibet.
"Meghan is not going to travel," an insider told the Daily Mirror. "She has just had a baby and Harry will travel alone."