Footage of the moment quickly appeared on Twitter, and the reaction from some was nothing short of hysterical.
Meghan's every smile, step and wave has been scrutinised by royalty watchers curious to see how the mixed-race American actress will perform in her new role.
Ah, yes - everyone loves a royal rebel ignoring the rule book.
They, however, soon found themselves on the receiving end of some comments from other royal experts - real ones, this time - who pointed out that they had got their facts wrong.
After getting out of the auto and shaking hands with one of the hosts, the duchess casually shut the door and went inside, writes People magazine.
Victoria Arbiter, 9Honey's royal commentator, perhaps summed it up best in her exasperated tweet.
The royal stepped out her vehicle, having her door opened for her, and was welcomed by Lord-Lieutenant of Greater London Sir Kenneth Olisa. "Auto door shutting does not fall under protocol!"
It's not the first time the Duke and Duchess of Sussex have travelled by public transport to an event, the couple also caught a train to their first official visit in Cardiff - which ended up being delayed.
Etiquette expert William Hanson confirmed this in an interview with a British radio show, explaining royals have members of staff to open and close auto doors for them as a security measure, rather than "airs and graces".
She went on to add that Meghan and Samantha - who wasn't invited to the wedding - have never been close, and probably never will be.
Since then the Duchess of Sussex has been making her own rules and living her life in her own way quite subtly.