The newly married couple travelled to their evening reception in a blue Jaguar.
Stella McCartney has revealed how she landed the commission to create the Duchess of Sussex's £60,000 (NZ$115,428) evening gown for her wedding day.
The British designer, who was awarded an OBE for her sustainable fashion label, says the pair have become good friends since Meghan began wearing her clothes following her engagement to Prince Harry, the Daily Mail reports.
Speaking to fashion bible Elle magazine, which next month focuses on sustainability, she said: "I think Meghan chose me for reasons other than just a beautiful dress — plenty of people can do that. She is not a difficult client to make look beautiful.
"Amal called me and asked me to do it, as did Oprah. I look at that Amal dress, and it is made of sustainable viscose that took us three years to develop."
She also says she believes that Meghan — along with her guests Amal Clooney and Oprah Winfrey — chose her designs because she is a strong feminist and environmental campaigner.
Stella McCartney (pictured at Berlin Fashion Week, June 2017) recently spoke of her 'pride' at making the stunning halterneck dress for Meghan and Harry's wedding reception. Photo / Getty Images
McCartney, 46, added: "And I think, obviously I am a British designer, but I think being a woman and being a women's woman played a part. They are all women's women. It's a big deal."
The designer, a lifelong vegetarian and animal rights campaigner, says she bonded with Meghan from the moment they met over their shared interest in environmental issues and the pair have since become friends.
She designed the £1350 (NZ$2597) black coat that Meghan wore to Wales in January; the £1512 (NZ$2908) dress she wore to the Royal Albert Hall in April and the £483 (NZ$929) silk blouse she wore for that month's Anzac Day service.
"[Meghan] approached me and we worked on other things," says Stella, who is married to publisher Alasdhair Willis, with four children Miller, 13, Bailey, 12, Beckett, ten, and Reiley, eight.
"She wore other pieces of mine and we had a friendship. And then she asked me to do her second wedding dress. I was honoured and I still am."
The stunning dress Amal Clooney wore to Meghan and Harry's wedding was also a Stella McCartney design. Photo / Getty Images
The British designer, who opened a new flagship shop in London this summer, also revealed how she tries to balance a focus on fair trade with being cost-efficient.
Although her parents Sir Paul and Linda McCartney have always supported her — they were in the front row of her graduate fashion show in 1995 — they did not pay her pocket money.
"Look I get it," she told Elle. "I really get it. I have struggled with the price point of my product from day one. I still do.
"I work so hard to keep it down; I get charged 30 per cent more tax when I take my non-leather products into the US; and I don't pass that on to my customer, I add it to my margin.
"I believe in quality, I believe in fairness, and the minute you pay someone fairly, your prices go up.
"I wasn't given money by my mum and dad, so I always shopped vintage at charity shops. And, damn it, I felt cooler for doing that.
"People know to do that now; there is enough visual collateral to give young people confidence to wear things in a way where they don't have to come to a fashion house to say who they are.
"Take ownership of who you are, take ownership of the environment you live in and be responsible. If you guys can't do it, then we are screwed."
Oprah Winfrey, dressed in a Stella McCartney-design, arrives at St George's Chapel in Windsor for the May wedding of Prince Harry and Meghan Markle. Photo / Getty Images
In June, McCartney spoke of her "pride" at being asked to make the stunning halterneck dress for the evening reception at Frogmore House — calling the commission "a big deal".
Speaking to the BBC, McCartney explained she was determined to get it right because the wedding reception would be Meghan's final appearance before taking on her "very austere, very serious" royal role as the Duchess of Sussex.
The show-stopping white halterneck gown was "the last moment" that Meghan, 36, could reflect "the human within her", McCartney added.
McCartney said the show-stopping white halterneck gown was 'the last moment' Meghan could reflect 'the human within her', before embarking on her life as a royal. Photo / Getty Images
While the neckline was modest, the backless number allowed Meghan to show off her toned shoulders and back, while the figure-hugging gown featured a dramatic fluted skirt.
The Elle Sustainability Issue goes on sale on Wednesday 8 August. Visit elle.com/uk
