Birmingham-born Clare Waight Keller also revealed that not even the people who worked on the dress knew it was for the now-Duchess of Sussex.
Nor did she tell her own husband and children until hours before the wedding, adding that it was “truly a secret Meghan and I shared”.
Ms Waight Keller, who made history as the first female artistic director at French fashion house Givenchy, was revealed as the designer of the eagerly awaited wedding dress on Saturday morning.
Describing it as an “enormous honour”, the 47-year-old said: “It was truly a privilege to work on this project.”
The bridal gown was simple in a classic white, but the elaborate veil was embroidered with flowers from each of the 53 Commonwealth nations.
Those working on it spent hundreds of hours meticulously sewing and washing their hands every 30 minutes to keep the tulle and threads pristine.
Ms Waight Keller also said Harry had approached her after the service.
She said: “He came straight up to me and he said, ‘Oh my God, thank you, she looks absolutely stunning.'”
Ms Waight Keller agreed, saying: “She just looked absolutely exquisite.”
Ms Waight Keller appeared on the steps of St George’s Chapel when Meghan arrived to ensure her veil and train looked pristine, and helped to arrange it before the couple made their exit as husband and wife – a role often performed by the maid of honour.
However, there was no maid of honour as Meghan did not want to choose just one person out of her close group of friends.
Ms Waight Keller said it was confirmed to her in early January that she was the chosen designer.
“It was an extraordinary moment when she told me,” she said.
“Of course it’s an incredible thing to be part of such a historic moment.”
She said the process was collaborative, but that Meghan had a “vision” for the dress.
Ms Waight Keller added: “I wanted her to feel absolutely incredible in the dress and also I wanted her to feel it was absolutely right for the occasion.”
The designer said: “She’s just so genuine and warm and radiant. She’s just glowing.
“She’s a strong woman. She knows what she wants, and it was really an absolute joy working with her.”
Asked about the moment Meghan was able to look at herself dressed and ready in the mirror on Saturday, Ms Waight Keller said: “She was just glowing. There’s so much emotion on a day like that anyway.
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“But I think particularly when it all comes together, I think it is tremendous. She was absolutely radiant.”
:: Relive the magic! See all of the guests as they arrived at the royal wedding – use our exclusive, interactive Who’s Who feature on the Sky News app and website. Watch our Sky News royal wedding hour-long special programme available on catch-up and Sky Q.