Mary Greenwell first began working with Diana when they teamed up in 1991 for Vogue's December issue, which was shot by Patrick Demarchelier.
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Princess Diana - one of the great beauty icons
"She was just coming into her own when we first met and had just married Prince Charles," the A-list make-up artist told Stylist.
"Her marriage seemed solid and she was just the most divine, sweet, kind lady. It was a huge privileged to work with our then future queen."
Mary revealed it was on their first shoot that she first convinced Diana to stop using blue eyeliner – "Blue eyes should never wear blue pencil or shadow" – adding, "From that moment on, she was open-minded about trying new looks and colours."
She said, "Diana was always very open and willing with her style. I think as soon as we worked together she realised the power of hair and make-up, and that is something that should never be underestimated. By anyone."
Princess Diana pictured in November 1995
Mary also spoke about one of Diana's greatest beauty assets – her flawless skin.
"Diana was pure magic and she had the most incredible skin, which frankly at the end of the day also can come down to family genes," she said.
"Diana was very sensible when it came to her skin, she would always make sure she took her make-up off at the end of the day and make sure she had a clean face before applying make-up.
"…After I started working with her, Diana did everything in moderation. She cut back on drinking so her skin was 100 per cent. Women today need to take care of themselves the same way. If you smoke, drink or do anything to excess, you will destroy your most important beauty asset – your skin."
Mary also revealed her beauty routine with the princess. "We always started with a moisturiser, a foundation to suit her skin tone exactly and then used a concealer to lift the eye area. Mascara – which is a gift of all gifts – and lip colour to suit her mood that day.
"I always taught her to make sure when applying mascara by herself that she covered the roots of the lashes too!"