The Norroy and Ulster King of Arms Robert Noel formally declared the accession of Charles III to the throne following the queen’s death.
Amid heavy rain, around 200 invited guests were inside the castle grounds, while hundreds more watched from under umbrellas outside the main gates.
Hillsborough Castle is the royal family’s official residence in Northern Ireland, and the Co Down village was recently honoured with a ‘Royal’ prefix in recognition of its long-standing regal links.
The ceremony began with the Royal Irish Regiment (RIR) band leading a procession of a Proclamation Guard from the 2nd Battalion of the RIR and representatives of Lisburn and Castlereagh Council to the front of the castle.
Prior to the proclamation, a 40-second fanfare was sounded by a bugler.
A bell in the clock tower of the castle’s Court House chimed once at 12pm to signal the start of the reading.
As Norroy and Ulster King of Arms, London-born Mr Noel is one of three senior officers of arms at the College of Arms in London.
Following the proclamation, a 21-gun salute was fired by the 206 (Ulster) Battery Royal Artillery.
After the last round, the bugler sounded a royal salute before the RIR band played a verse of God Save The King.
Mr Noel then invited guests to cheer three times for Charles.
Guests included Northern Ireland Secretary Chris Heaton-Harris and Northern Ireland Office minister Steve Baker.
DUP leader Sir Jeffrey Donaldson, Alliance Party leader Naomi Long, Ulster Unionist leader Doug Beattie and Traditional Unionist Voice leader Jim Allister were also there.
After the short service concluded, the bells from St Malachy’s Parish rang out across the village.
The armed forces played a major role in the ceremonial activities, with senior figures from the Royal Navy, Army and RAF in attendance.
Commodore Kris Nicholson from the Royal Navy represented the First Sea Lord at the service.
He told the PA news agency: “It was a huge privilege.
“A huge honour and one of the highlights on the ceremonial side of my career. This is my first time here and I was hugely impressed.
“I understand there has been renovations recently, and the grounds, the inside of the castle, it is superb and reflects extremely well on the occasion.”
Brigadier James Senior, Commander of the 38 (Irish) Brigade, represented the Army.
He said: “It is a great moment of solemnity as we reflect on Her Majesty’s seven decades of service, but then also a moment of change for our country as we look forward to the reign of King Charles.
“As well as these fantastic public buildings, there is a very warm and deep sense of affection for and between the people of Northern Ireland and our late Queen. But then also similarly with His Majesty.”
Air Officer NI Marshall Sean Reynolds is the RAF Chief of Air Staff’s senior representative in Northern Ireland.
He said: “It is a moment of sadness as we reflect on Her Majesty.
“I was lucky enough to get to meet her on a number of occasions and she had that unique ability to make you feel like you were the only person in the world when you spoke to her.
“But we are also welcoming the new monarch, His Majesty the King.
“There is a huge amount of pride in how the armed forces managed to do this so well.
“My hat’s off to the Royal Artillery for a magnificent gun salute to mark the occasion.”
As well as the VIP guests, more than 100 residents of Hillsborough were invited inside the palace grounds to watch the ceremony.
Pauline Hampton and Collette McGowan, who both live in the village, spoke of their pride at receiving an invitation.
Ms Hampton said: “I ring the church bells in St Malachy’s, Hillsborough Parish Church, and an invitation was kindly extended of which I accepted.
“It meant everything to me to be here today. It truly was a momentous occasion.
“The weather certainly did not put us off. We would have been here no matter what the weather.
“I was devastated when I heard the news about the Queen, we knew at lunchtime that she wasn’t well. It was just devastating when the news came through at teatime.”
When asked about the new king, she said: “Charles has had a good teacher, I don’t think he will go too far wrong.”
Ms McGowan added: “I think Charles will do a great job.
“He has watched what his mother has done for the last 70 years and I think he will make a good king.”
The ceremony in the small Co Down comes ahead of Charles’ first visit to Northern Ireland on Tuesday, when he will travel to Hillsborough Castle to view an exhibition on the Queen’s long association with Northern Ireland.