For more than 20 years, he has been entrusted by Prince Charles to act as an ‘older brother’ figure to his two sons, dutifully extracting Harry from nightclub scrapes or advising both of them on their military careers.
Now former Welsh Guards officer Mark Dyer has a new role, The Mail on Sunday can reveal, acting as a pivotal go-between in the sensitive business of building bridges between Harry and William, particularly since the younger sibling turned his back on Royal life to live in Los Angeles.
This is a huge task. It is well established that misunderstandings, perceived snubs and pettiness on both sides caused a tragic rift between the once-close brothers.
Having served in the Army during the Troubles in Northern Ireland, peacekeeping is second nature to Dyer. The Princes are pictured above in a Commonwealth Day service in March
The trigger was William’s advice to Harry to ‘take things slow’ when he first dated Meghan Markle.
And relations became so sour that they reportedly didn’t talk for weeks after Harry’s wedding. Then Megxit, and the way it was handled, left William and Charles devastated.
Having served in the Army during the Troubles in Northern Ireland, peacekeeping is second nature to Dyer.
A friend says: ‘Long transatlantic chats between Mark and Harry have started to reap healing dividends between the brothers. His advice to the Prince was ‘talk and listen’.’
Of course, that’s not always been easy, as the friend explains: ‘Both William and Harry are proud and angry.
‘Both felt let down by each other. William felt that Harry had deserted his family and his duty. Harry felt that William and Kate hadn’t welcomed Meghan into the fold.
Former Welsh Guards officer Mark Dyer has a new role, The Mail on Sunday can reveal, acting as a pivotal go-between in the sensitive business of building bridges between Harry and William. Mr Dyer and Prince Harry are pictured in 2016
‘However, both realised what was at stake and that they were in danger of losing each other. They are now talking, but it will take time to mend.’
Cheltenham College-educated Dyer, 53, known as Marko to friends, became a mentor to teenage Harry and William following the death of their mother, Princess Diana.
He helped plan gap years abroad, attended Harry’s Sandhurst passing-out ceremonies and was a driving force in establishing his Sentebale charity in Africa.
Described as ‘one of the few people who talks some sense into Harry’, he is godfather to the Prince’s son Archie, while in turn Harry is godfather to Dyer’s eight-year-son, Jasper, who was a pageboy at the Sussexes’ wedding.
After leaving the Army and working as Charles’s equerry, Dyer set up some London gastropubs, including The Sands End in Fulham, where Harry conducted his secret courtship with Meghan.
The trigger was William’s advice to Harry to ‘take things slow’ when he first dated Meghan Markle. And relations became so sour that they reportedly didn’t talk for weeks after Harry’s wedding. Then Megxit, and the way it was handled, left William and Charles devastated
He now runs the Brook House in Parsons Green, where he dined with Harry the night before the Megxit deal was announced in January. Crucially, too, in terms of his rapprochement role, Dyer’s Texan wife Amanda, the daughter of a managing director of JP Morgan Securities, is liked by Meghan.
She is said to have taken the actress under her wing, knowing first-hand what it is to marry into the British Establishment.
The friend adds: ‘Mark has never let the boys down. Privately, he can be an honest critic but he’s rock-solid in times of trouble.’
Unlike many others of Harry’s old set, Dyer survived Meghan’s ‘shuffling’ of the pack of her husband’s friends. Make no mistake, there is still a long way to go to bring the Princes together, but Dyer commands Harry’s respect and knows more intimately than most the details of how the brothers fell out.
Certainly, Dyer will have told Harry that William’s advice about his relationship with Meghan when it was at an early stage was not about her character but more about Harry’s impetuous nature. Their family – on both Windsor and Spencer sides – is full of broken marriages and William was just advising caution.
After leaving the Army and working as Charles’s equerry, Dyer set up some London gastropubs, including The Sands End in Fulham, where Harry conducted his secret courtship with Meghan. He is pictured above with Prince William in 2005
After Harry took the counsel badly, William turned to Diana’s younger brother Charles Spencer and he agreed to raise the subject with Harry. But he, too, was snubbed.
Other senior members of the family also ‘had concerns’. It was said that Prince Philip commented with characteristic bluntness: ‘One steps out with actresses. One doesn’t marry them.’
Matters were further complicated by plans for a 2017 ceremony at Althorp, the Spencer family home in Northamptonshire, to commemorate Princess Diana 20 years after her death.
William and Earl Spencer had organised the private service, con-ducted by the Archbishop of Canterbury Justin Welby, for close family, including Diana’s sisters. William wanted Prince George, then four, and Princess Charlotte, two, to attend.
By this point, Harry had already discussed marriage with Meghan and he wanted her to be with him. She travelled from Canada to Britain to be there but did not, in fact, attend. At the time, it was explained that her presence was not considered ‘appropriate’.
An upset Harry told Meghan of his family’s ‘warnings’ and a very definite sense of ‘us against the world’ became entrenched.
The couple felt that William and Kate had not ‘rolled out the red carpet’ for Meghan, who had hoped she would have an ally in the Duchess of Cambridge, a fellow outsider who had navigated the choppy waters of Royal life with aplomb. But the naturally cautious and private Kate did not respond to her enthusiastic overtures of friendship.
One incident illustrates one of the many minor misunderstandings that drove a wedge between the two families. Kate likes to pop out to shop discreetly on the Kings Road in Chelsea, being driven by a protection officer.
But on learning that Meghan was planning a trip there, too, on the same day, Kate did not offer a lift – deciding that two of the world’s most famous women going shopping together might cause a scrum of paparazzi. Meghan saw this as a snub and a sign that she would never be welcome.
Then she learnt the Middletons were nervous about inviting her to Pippa’s wedding, fearing her first public appearance with Harry might overshadow the bride’s big day.
Eventually she was asked, but went to the evening party only. By now, a pattern was emerging.
Tensions heightened during the planning for Harry and Meghan’s wedding, often over little things.
Palace staff reported tantrums about the choice of tiara, requests to spray the Windsor chapel with French perfumier Diptyque’s scent, and bossy instructions sent at 5am by email.
Another sore subject was the question of a pre-nuptial agreement, with Harry vehemently resisting any suggestion of one.
And their perception of senior Royals pulling rank certainly rankled with Harry and Meghan.
Society bible Tatler last week reported that Meghan and Kate also fell out over whether bridesmaid Princess Charlotte should wear tights, with her mother unsuccessfully insisting it was protocol not to be bare-legged.
Kensington Palace denied this, saying the story was totally false.
What actually happened was slightly different. At a bridesmaid dress fitting, Kate was unhappy that the length of Charlotte’s hem was too short. She asked Meghan to organise a correction but the resulting interaction left Kate in tears.
That said, the atmosphere is colder between the two husbands than the wives.
The couple felt that William and Kate had not ‘rolled out the red carpet’ for Meghan, who had hoped she would have an ally in the Duchess of Cambridge, a fellow outsider who had navigated the choppy waters of Royal life with aplomb
I have been told by a courtier: ‘The two Duchesses are not best friends but they are professional. They do text each other. But they are very different characters. Catherine has tried to broker peace between the brothers as she knows how important their relationship is to both the family and the Monarchy.’
Perhaps there was something inevitable about William and Harry going their separate ways.
Since the day he was born 38 years ago next month, William has always been second in line to the throne. Whereas Harry, 35, has taken positions such as president of the Queen’s Commonwealth Trust.
As one friend of William puts it: ‘Diana brought both boys up to be equal and that was all well and good, but they were and never will be equal.
‘William will be King. Harry won’t. Harry often finds it hard to accept that he isn’t as important as his older brother.’
An aide added: ‘William is very concerned with both his image and ‘Brand Cambridge’, although he’d never term it like that, and how in turn that may reflect on the Monarchy’s popularity as a whole.’
Certainly, that imperative has become much more important given the way his brother has walked out on the Royal Family.
Even so, there is still much to unite the Princes, which Mark Dyer will use in his bridge-building.
For example, both men have a passion about highlighting the importance of mental health.
Last week, William featured in a BBC1 documentary on the subject, while Harry is working with Oprah Winfrey on a mental-health documentary series for Apple TV.
How sadly ironic, therefore, that some members of the Royal Family have been worried about the mental health of both Meghan and Harry.
It was significant that while on a tour of Africa after becoming a mother, a plaintive Meghan complained that ‘not many people have asked if I’m OK’.
If anyone can bring the Princes back together, it’s Mark Dyer, the man who Royal biographer Penny Junor said ‘stepped in after Diana’s death to be the glamorous, daring big brother Charles’s sons never had. The boys adored him and they are all firm friends to this day’.
Dyer has one other important link with the pair: he once briefly dated the other person Charles used ‘in loco parentis’ after Diana’s death, their trusty nanny Tiggy Legge-Bourke.
He also shares their love of Africa, having proposed to his future wife on holiday in Botswana while camping beside the Boteti River in the heart of the bush.
Over the coming months, he will need to draw on every drop of empathy to fulfil his role as peacemaker.