"The Duchess of Sussex closing her own car door is yet another silent signal that she is going to be doing things her own way and helping adapt and progress the British monarchy," he said.
"Whether she thought long and hard about it before it happened, I doubt it, I suspect it was force of habit, but it is interesting to see that many have noticed as usually dignitaries have a member of staff to do this for them.
"That comes from a security point of view, mainly, not just for perpetuating airs and graces."
However he noted: "There would have been little security bother last night with Meghan closing her own door as she exited the car on the side closest to the Royal Academy."
The duchess was on hand to open the "Oceania" exhibition in her first solo outing as a member of the Royal Family.
She wore a £2,510 Givenchy dress and a £1,390 box clutch by the designer for the occasion.
Christopher Wilson, author of titles including The Windsor Knot, Fergie and Charles V Diana, was among the first to note the break from royal protocol.
He tweeted: "First time I've seen an on-duty princess shut her own car door..."
Followers commented on the significance of the moment, with one replying: "Breaking all stereotypes I love this new generation."
Another added: "I don't think #MeghanMarkle sees herself as better than anyone else. She knows that she and #PrineHarry are blessed to have found each other. I'm proud of her. Life is good."
A handful of eagle-eyed royal fans pointed out the duchess had also closed her own car door on arriving at the Together cookbook launch at Kensington Palace last Thursday.
Arriving with her mother Doria Ragland and husband Prince Harry Meghan climbed out of the car and closed the door behind her.