Members of the 206 Ulster Battery of the Royal Artillery fired from three guns outside the castle as hundreds of people gathered in the grounds of the royal palace to watch the commemorative event.
Among guests were Northern Ireland Secretary Brandon Lewis and former Stormont first minister Paul Givan.
Later on Thursday, beacons will be lit across the region, including on Northern Ireland’s highest peak and in the UK’s most westerly town.
The official four-day celebration of the Queen’s 70 years on the throne will include street parties, exhibitions, church services and lunch events.
Speaking at Hillsborough Castle, Mr Lewis said the jubilee provided a chance for all communities across the UK to come together.
“This is the start of four days of celebration of one of the most amazing women that we have seen in public service around the world,” he said.
“A Platinum Jubilee is not something that happens very often and the service that Her Majesty the Queen has given to both the UK and the Commonwealth more widely is being recognised, and I think it is also a great opportunity for people across all communities across the UK to come together, to spend time together and to celebrate an amazing lady.”
He praised the Queen for always taking a “proactive” interest in Northern Ireland.
“I think it is really good that a wide range of people, all of us, can recognise and celebrate that service,” he said.
“When I talk about that service Her Majesty has given to the country, it is not just the stability of being in place and serving the Commonwealth and the UK, but actually taking a proactive interest in and role in some of the most important things in the history of our country.
“The peace process in Northern Ireland is a very good example of that and I think it is something we can all be very proud of, having somebody as our Queen who has been through that process with us and been steadfast in doing so as well.”
Mr Givan said the 42-gun salute at Royal Hillsborough was “very special”.
“Never again in my lifetime are we going to get a Platinum Jubilee, maybe never again in history will we get one, so this is a hugely important event,” he said.
“The public are really embracing this occasion to celebrate the Queen’s Platinum Jubilee celebrations and it’s fantastic to be part of it.”
Thursday marks the start of a long bank holiday weekend in Northern Ireland which will also include a visit from the Earl and Countess of Wessex.
The royal couple are expected to carry out two engagements in the region on Saturday.
As well as the gun salute at Royal Hillsborough, which is the Queen’s official residence in Northern Ireland, in Belfast the HMS Pembroke will be docked for the weekend and open to visitors with pre-booked tickets.
The UK’s most westerly town, Enniskillen in Co Fermanagh, will enjoy one of the busiest celebrations.
Visitors can enjoy a special exhibition at Enniskillen Castle which will include a maid of honour gown and page outfit worn by local people who played key roles at the coronation.
It will also host a special church service on Thursday evening at St Macartin’s Cathedral, where the Queen visited in 2012 during her Diamond Jubilee, before beacons are lit.
As part of 1,500 beacons being lit across the UK between 9.15pm and 9.45pm, many locations in Northern Ireland will host a lighting including Titanic Belfast in the capital city and St Columb’s Cathedral in Londonderry as well as the region’s highest peak.
The Walking With The Wounded charity will hike a beacon up Slieve Donard in Co Down, while their colleagues do the same at the other UK peaks, Ben Nevis, Scafell Pike and Mount Snowdon.
At the weekend, big lunch events will be taking place across the region, including on the Shankill Road and in Bangor, Co Down, which was made a city as part of the jubilee celebrations.