This week, Michelle Obama flew into Stansted on a solo visit to Europe, while her husband stayed in Washington. Eagle-eyed viewers might have noticed that her arrival at London's least glamorous airport was not entirely solitary. The First Lady was accompanied by her daughters, Malia (16), and Sasha (13) - and her mother.
Marian Robinson has no official job title, but in Washington she is referred to as Grandmother-in-Chief, or First Grandmother of the United States (FGOTUS, pronounced fuh-GOH-tus - a riff on POTUS, which is shorthand for her son-in-law's position as President of the United States).
They may be jocular, but these nicknames hint at Mrs Robinson's significant influence within the First Family. She is not just along for the ride; she is very much a key part of the Obama entourage.
"Michelle calls Marian Robinson her 'rock'. She is steady, she is strong," says Peter Slevin, whose biography Michelle Obama: A Life, has just been published.
"She has opinions, but she is very grounded in the real world," he says.
A sign of her importance is where she lives. Namely, one floor above Barack and Michelle Obama. While the President and his immediate family live on the second floor of the Residency of the White House, his mother-in-law lives on the third.
The idea of holding down the most important job in the world while having your mother-in-law living above the shop has been a gift to satirists and Obama's critics.
But it is not unprecedented: Harry Truman's mother-in-law moved in, despite her dislike for Truman, while Woodrow Wilson's second wife had her mother installed.
If anything, Marian Robinson's presence at the White House reflects the increasing youth of today's world leaders. Both Margaret Thatcher and Ronald Reagan were grandparents while they were in power.
But this sudden lurch youthwards means that many world leaders are now the classic sandwich generation: bringing up children, while caring for their elderly parents. Mrs Robinson is 77, but very fit and active. Indeed, back in 1997 she won the gold medal in both the 50 and 100 metres at the Illinois Senior Olympics. She abandoned the sport after an injury.
"If I can't do it fast, I'm not doing it," she said in Oprah Winfrey's magazine, O. "You don't run just to be running - you run to win."
You can see where her daughter gets her drive from.
After a lifetime spent in Chicago, Mrs Robinson had to be persuaded to join the First Family in Washington when Obama won his first election in 2008. Although her husband, Fraser, had died in 1991, she was reluctant to leave her modest bungalow, siblings and friends. But she had already become a vital part of the Obama support network, having given up her job as a secretary at a bank so she could help out with child care during the presidential campaign.
After the election, she agreed to visit for three months to help Michelle, and particularly the First Daughters, settle down.
"They're dragging me with them, and I'm not comfortable with that," she told CBS at the time. "But I'm doing exactly what you do. You do what needs to be done."
Six-and-a-half years later, she is very much at home in Washington. Not only has she made new friends - notably Betty Currie, the former private secretary to Bill Clinton - but created her own life.
Being able to slip out from the White House without secret service protection means she can regularly attend performances at the Washington Centre, for instance.
"It's important to remember the White House is such a bizarre place. The Obamas have 24-hour security, hot and cold running staff, they are under a microscope," says Slevin.
"I think Mrs Robinson acts as a calming presence. Barack Obama talks about how much he enjoys having her in the White House. He also says because she can slip out, she comes back to the White House with stories of real life. She is a humanising presence."
Now that the girls are growing up, and her regular duties of picking them up from school and reading bedtime stories are coming to an end, her role has changed into something even more important: chaperoning the girls on global trips.
Before her son-in-law entered the White House, Mrs Robinson had never been abroad. Now, accompanying her granddaughters, she has visited Senegal, Tanzania, China and Italy, where she met the Pope.
That is some journey, considering that she was born, one of seven children, in 1937 in segregated Chicago. She did not go to college and married Fraser C Robinson III, a former boxer and soldier, who spent his working life tending boilers at the city's water plant. They brought up their children, Michelle and Craig, in a modest home in the working-class, south side area of the city. Until the children left the nest, she was a stay-at-home mother, obsessed with them getting in to the best colleges. Both went to Princeton.
In a rare interview, given to CBS, Mrs Robinson denied she had some magic dust that meant her daughter ended up in the White House, and her son was a successful college basketball coach. "I don't think it was magic dust, as a matter of fact. I believe there were lots of families who did exactly the same thing. But that was just the norm - you raised your children to stress education. We wanted them to have the feeling they could try anything. The saying was: 'If it can be done, you can do it.' It was a matter of choice."
As Obama's presidency starts to wind down, it is unclear how history will view his two terms in office. The conclusions are likely to be mixed. The First Lady, however, has scored some notable hits, not just with her anti-obesity and pro-education drives. More than anything, she has held her family together with a semblance of normality in the goldfish bowl of the White House.
And much of that success is thanks to the Grandmother-in-Chief.