The former Great British Bake Off judge, 89, was photographed in the Bridgerton garden, joined by Hannah Dodd, who plays Francesca Bridgerton in the series, and Ruth Gemmell who portrays her mother, Lady Violet.
The garden is inspired by Penelope Featherington, played by Derry Girls star Nicola Coughlan, whose love story is one of the main focal points in the romance series’ latest season.
Dame Mary also visited the National Garden Scheme garden, joined by the Chelsea Pensioners, who are retired soldiers of the British Army.
The pensioners are based at the Royal Hospital Chelsea, where the annual flower show takes place.
The TV chef is the president of the scheme, which gives people access to over 3,500 private gardens in England, Wales and Northern Ireland.
She was pictured in the garden wearing a pink, green and blue dress alongside gardener and broadcaster Alan Titchmarsh – another supporter of the scheme.
Elsewhere, Dame Judi Dench was seen meeting naturalist and TV presenter Chris Packham at the garden show.
The veteran actress, 89, who is known for her love of nature and helmed a 2017 BBC documentary called My Passion For Trees, wore a colourful jacket and black sunglasses to the annual event.
The Chelsea Flower Show regular also visited the Octavia Hill Garden, sponsored by Blue Diamond Garden Centres and the National Trust.
She was photographed with a seedling in a pot from the Sycamore Gap tree, which was a much-photographed tree, chopped down in September last year.
Another visitor to the site was Dame Joan Collins who cut an elegant figure in black sunglasses and a blush-coloured summer hat and jacket which had a flower fastened on it.
The 90-year-old veteran actress appeared alongside her husband – producer and actor Percy Gibson.
Game Of Thrones actress Indira Varma unveiled the WaterAid Garden at the flower show and posed with an umbrella made from climate-resilient flowers.
The garden aims to shine a light on the global water crisis with its flood and drought resilient plants which can adapt to a changing climate.
The centrepiece of the garden is a 3.6 metre tall rainwater-harvesting pavilion that is inspired by the charity’s work to help provide access to clean water across the globe.
Former Countdown star Carol Vorderman batted off the sun with some wide rimmed sunglasses paired with a denim jumpsuit and cream coloured handbag.
Elsewhere Dame Joanna Lumley was seen in a stylish blue hat, joined by The Great British Bake Off judge Dame Prue Leith and celebrity chef Marcus Wareing.
Other celebrity attendees included TV presenter Nick Knowles and his fiancee Katie Dadzie, Good Morning Britain’s Charlotte Hawkins, sports presenter Gabby Logan, Britain’s Got Talent judge Bruno Tonioli and Wicked Little Letters star Joanna Scanlan.
The Gentlemen star Joely Richardson also attended and was photographed in the Burncoose Nurseries Garden.
The world famous show opens to the public on Tuesday.