LONDON — Lady Elizabeth Anson, Queen Elizabeth II’s cousin and her party planner for more than 50 years, was raised in a Downton Abbey-like home where servants ironed the newspapers, and was married at Westminster Abbey.

But on a recent Friday, Lady Elizabeth — the orchestrator of some of Britain’s most aristocratic parties — was in her white-azalea-filled sitting room. She was wearing a gray Tomasz Starzewski jacket bought on sale more than 40 years ago, drinking tea from a purple-flowered tankard (an eBay find) and discussing her recovery from her weekly lottery ticket-buying addiction.

“I still do a lucky dip if it’s a huge amount of money,” said Lady Elizabeth, who has also organized events for Tom Cruise and other celebrities seeking discretion.

On her feet were what looked like Roger Vivier court shoes, but when a reporter suggested she might like to remove whatever was stuck to the bottom of them before having her photograph taken, Lady Elizabeth, 75, handed the footwear to her liveried butler. “Certainly, my lady,” he said, before getting rid of the price tags that revealed that the shoes cost £25 (about $35).

“Marks & Spencer’s best,” Lady Elizabeth said, looking delighted at this evidence of her frugality.

And really, why shouldn’t she be? Among her “grander family,” as she calls them, frugality is a trait to be celebrated. The queen has been known to take public trains. Kate Middleton, the Duchess of Cambridge (whom Lady Elizabeth refers to as “Catherine”), is applauded for wearing dresses more than once.

Let somebody else be the embarrassingly profligate relative that makes the monarchy look bad. Lady Elizabeth — daughter of Princess Anne of Denmark, goddaughter of King George VI — is just another hard-working entrepreneur, albeit possibly the only one with the word “lady” in her email address.

“I do not know how to be a lady who lunches,” she said. “I love my work, and my work pays for my home.”

She asked the butler to bring her her nicotine sweets. She gave up smoking in the ’60s, picked it up after 17 years, then quit with difficulty again 11 years ago.

If she were to start again, the sitting room would rebuke her. Every surface is crammed with decorative eggs — 536 of them, by her grandchildren’s last count — that she began collecting after her stepfather presented her with an alabaster one as a worry bead for her right hand when she first put down the cigarettes.

Hers is a nerve-racking business. One of her projects is the queen’s 90th birthday this month. The milestone is being marked with street parties, pageants and ceremonies, but Lady Elizabeth handled the private family celebration, which, like all of her events, she was “not prepared to discuss.”

Still, she is as unguarded as one can possibly be when bound by confidentiality agreements, referring to one of Ivana Trump’s post-Donald husbands as “looking like a frog.” (Mrs. Trump is a former client. Things did not end well.)

Of the overweight king of Tonga, whom she was looking after, along with all the other foreign heads of state, at the 2011 royal wedding, she said, “He thought he’d break the furniture in Buckingham Palace.” The chairs there are “very low,” she said, “very wonderful Louis Quatorze. It looks as if you sat on it, it would break. But it doesn’t.”

During a discussion about the lost art of conversation because of cellphones, she took her incessantly ringing land line off the hook, letting the receiver dangle at her stockinged feet, and leaned in, saying: “I think I can tell this. It’s a bit about the royal family.”

She described how the queen had had her grandchildren over for dinner. “And she said to me that she found it really difficult,” Lady Elizabeth said, “because they didn’t really know how to talk each other. And she said, ‘I suppose it’s because they’re always getting up and down and helping somebody and putting something in a dishwasher or whatever they’re doing, because they don’t have enough staff.’”

On the subject of Her Majesty: For the record, the queen is a “most meticulous hostess,” though she does not insist only on Malvern water or yellow freesias, as has been reported. “People love creating myths,” Lady Elizabeth said. “It makes them feel very self-important.” In fact, it was a gardener at Windsor Castle who decreed the yellow flowers.

For the wedding of William and Kate, for which Lady Elizabeth was an adviser (and planned the wedding eve family dinner), the couple had very strong ideas that the queen “was marvelous and listened to,” she said. Traditional royal weddings have no reception afterward — guests just depart after the service — but William and Kate wanted one after seeing that was what their friends had had.

“And so the palace had to learn quite a lot about different canapés, because they weren’t used to doing them,” Lady Elizabeth said.

Her code name around the office for the queen is (or was until it’s been published) Shirley Temple, though Lady Elizabeth said she can’t remember why. Referring to her staff of five, she said: “We literally had the funniest names possible for everybody. And the people who were the most aware of their celeb importance that I’ve ever had got names that were truly extraordinary.”

While looking at wedding sites with the actress Isla Fisher, who was marrying Sacha Baron Cohen, Lady Elizabeth was amused that Ms. Fisher (code name: Sally Dangletrot) kept three different wigs in the car to disguise herself. “I wouldn’t have known her if she was passing down the street,” she said. “I shouldn’t say that because it’s quite rude.”

Lady Elizabeth started her business at age 18, in 1960. Her inspiration was the stress of planning her own 1959 debutante party, which prevented her from enjoying the evening.

Chasing down R.S.V.P.s was a particular headache, and she claims credit for inventing “this ghastly thing called the reply card” to solve the problem. The first time she created one, she forgot to leave a space for people to write their names, and so she ended up with a bunch of yeses and nos but no clue who they were from.

Her first event for the queen was a disco at Windsor Castle for Prince Charles, then 15, and Princess Anne, then 13. “My business started before the discothèque was invented,” she said. “So when this man told me he was going to charge me £25 to put on records for the evening, I thought, ‘Is this man absolutely crazy?’ Anybody can put on a gramophone record.” She soon learned it was “an art form.”

A party for the Rolling Stones ended with the police showing up as guests were drunkenly throwing unopened bottles of Dom Pérignon into the Thames, but generally Lady Elizabeth’s events are known for their calm elegance and thoughtful touches.

At a white-themed dinner for 40 people last month hosted by royalty at the cavernous, drafty Victoria & Albert Museum, Lady Elizabeth marched over to “my little Indian caftan man on the Portobello Road” and bought a load of white pashminas to drape on each chair. She hid hot-water bottles underneath cushions. (She also suggested to guests that they wear “heat tech from Uniqlo.”)

“It’s that ghastly boring phrase ‘the devil’s in the details.’” she said. “The devil is in the detail, and the detail doesn’t need to cost very much.”

She can do strict protocol — working out what year various marquesses or earls were created to determine seniority (and thus seating arrangements) — but she is not afraid to break it, along with convention.

Glynn Woodin, who has worked with her for 30 years as the managing director of Mustard Catering, a society favorite, recalled that recently she decided the damask tablecloth (Mr. Woodin described its color as “eau de vie with a bit of olive”) would look better flipped upside down, using the rougher textured side. “We were doing a party absolutely littered with royals,” he said. “You and I would be horrified to use a cloth on the wrong side, but it was exactly right.”

She has never advertised. Instead, she attributes her success in part to being “terribly, terribly shy” — she still bemoans a party she never quite made it to years ago where, dressed in a red velvet trouser suit and with her hair freshly done, she stood by the elevator watching people go in, unable to summon the nerve to do the same.

And so she arranges party spaces with the timid in mind. “The downfall of any party in the countryside is to walk into a hall and be confronted by a dance floor,” she said. “And some young man has driven you down, so he’s got a girl on both arms, and what do you do with yourself?” Her solution: a well-lit bar, which she calls a “picking up and dumping ground.”

With multiple events a week, she said she has little time for hobbies, though she was an avid “Downton Abbey” watcher (dismayed as she was by the dining room tablecloth — “a well-polished table was a butler’s pride and joy”). She has a fondness for foraging, especially for mushrooms. “There’s a wonderful book called ‘Food for Free,’ and it’s quite amazing what you can eat from the hedgerows,” she said.

She paused to contemplate what she would do if she retired, but then quickly got back to work.