The three generations of royals were all smiles as they arrived at the 305-year-old store together in the queen's Bentley to celebrate London's Piccadilly shopping district, including the reopening of Fortnum & Mason's restaurant, renamed The Diamond Jubilee Tea Station in honor of the queen's 60 years on the throne.
The visit made history as the first time Middleton, 30, joined the queen for an engagement without her husband, Prince William, who is away on a six-week tour of duty in the Falkland Islands.
The Duchess of Cambridge, her mother-in-law and grandmother-in-law joined 150 of the store's staff members at a tea party and were presented with Fortnum's famous "hampers," or baskets, stocked full of treats for them and their mutual affection, dogs.
"There was lots of joy, especially over the biscuits," People magazine reported, citing a royal source. "The [Queen's] Corgis are going to be lucky tonight. They'll be in doggie heaven."
The tea stop is the first of four visits the 85-year-old queen has personally invited Middleton to attend as she embarks on her Diamond Jubilee Tour celebrating her 60 year anniversary.
"The Queen is actually taken Kate under her wing…and that's very different because that did not happen within the first year of marriage for Diana or for Fergie," Michelle Tan, senior editor for People magazine, told " Good Morning America," referring to the former wives of Princes Charles and Prince Andrew, respectively.
While all dressed in shades of blue, the three generations of royals still showed their generational differences in style. The queen donned her typical two-piece coat dress with military-style rows of buttons and her signature hat while Middleton stepped out in an above-the-knee tweed coat dress by designer Missoni and gray suede heels by Rupert Sanderson. She also chose to accent her coat with daffodils to mark St. David's Day. Camilla wore a long, navy coat with white stripes by Bruce Oldfield.
Middleton and Prince William have made it a mark of their nearly one-year marriage to maintain a sense of normalcy, eschewing a heavy staff and even grocery shopping on their own, and Middleton's comments at the store seemed in line with that approach.
"I would really like to learn to make the perfect cup of tea, as when I last made a pot of tea with dried tea leaves I got it very wrong," Middleton told a Fortnum employee, according to People.
She also lingered at the store's chocolate counter and was overheard telling staff, "I love chocolate. I think all girls like chocolate."
Next week Middleton will again join the queen at a Diamond Jubilee engagement, this one in the English town of Leicester where Middleton will announce which of six local students she's chosen to design a custom-made pair of shoes for her.