Dubbed "the most beautiful member of the Royal Family" by Tatler, Amelia, 21, appears in a promotional short film shot at the El Rocío pilgrimage – a huge annual spring festival that attracts in Andalucia, Spain.
"I invited her to join us at El Rocío because I knew she would genuinely enjoy the experience," explained designer Penelope, whose footwear has long been a hit with Kate – the duchess wore a pair of the brand's boots on last year’s royal tour of Bhutan.
Amelia – who is 36th in line to the throne – is pictured in the short film enjoying the sights and sounds of Spain before slipping into local fashion, including a pair of Penelope's own worn-in boots, to join the festivities.
Speaking to The Telegraph, Penelope praised Lady Amelia for "immersing herself in the local culture" before working on set. She also revealed that although they were in Spain to work, they made sure to explore everything that Andalucia had to offer.
"We went into the back parlour of the market stalls to have dresses pinned, listened to gossip in the local hairdressers and ate in bars signed, 'Reserved for Horses' - where horses literally walked into. It was incredibly fun!" the designer told the publication.
She's not the first royal to work with a fashion label. Zara Tindall partnered with British brand Musto, Charlotte Casiraghi of Monaco has modelled for Gucci and Chanel, and Princess Olympia of Greece fronts Michael Kors' "The Walk" campaign.