David Anderson, the former household manager the Queen's official residence in Northern Ireland, has died following a short illness. he was 58.
The father-of-three held the post at Hillsborough Castle for 25 years during many key political moments.
He served many guests at Hillsborough Castle, including the Queen, four prime ministers and 12 secretaries of state.
He was awarded an MBE and was also made a member of the Victorian Order - to mark his personal service to the Queen.
In recent years Mr Anderson worked at Montalto House in Ballynahinch.
Speaking to the BBC during the Queen's Diamond Jubilee tour in 2012, Mr Anderson said the Queen's only request during visits was "somewhere quiet, somewhere private, and somewhere comfortable to sit in between engagements".
While Hillsborough Castle is home to the Royal family during visits to Northern Ireland, it is also the residence of the Secretary of State.
Shaun Woodward, a former Labour Secretary of State, described Mr Anderson as "the embodiment of decency".
"He was an extraordinary man, a loyal colleague and a true friend, loved by his friends. He was an exceptional man.
"He was able to make Hillsborough Castle a home to families, like myself, when I was living there.
"David was able to keep the machine oiled so people could be there, work there, live there and make agreements there," he said.
During a quarter of a century at Hillsborough Castle Mr Anderson witnessed numerous political agreements in Northern Ireland's history, including the signing of the Anglo-Irish Agreement in 1985.
"He was witness to things going well, but also to things not going well," said Mr Woodward.
"Every Secretary of State trusted him, not only in the personal sense with their lives, but they trusted him as the keeper of secrets as we pushed agreements forward."