Situated on the east shore of Strangford Lough, a few miles outside the town of Newtownards and near Greyabbey, it was the Irish seat of the Stewart family, Marquesses of Londonderry.

Prominently associated with the 2nd Marquess, Robert Stewart, Viscount Castlereagh, Britain's Foreign Secretary at the Congress of Vienna and with Charles Vane-Tempest-Stewart, 7th Marquess of Londonderry, the former Air Minister who at Mount Stewart attempted private diplomacy with Hitler's Germany, the house and its contents reflect the history of the family's leading role in social and political life in Britain and Ireland.

They were Presbyterians, farmers and linen merchants whose fortunes had been transformed by Alexander's marriage to the sister and heiress of Robert Cowan, the East India Company governor of Bombay.

In the increasingly troubled 1790s, Mount Stewart quietly converted to Anglicanism and stilled the contest, agreeing with Hillsborough that each should return a member to the parliament in Dublin unopposed.

[5] In 1787, writing to her brother William Drennan (a disappointed supporter of the Stewarts' electoral ambitions, later to be targeted by Castlereagh as a United Irishman), Martha McTier described visiting Mount Stewart, and meeting "with no one thing worth notice, unless great wall pounds are so – much expense, no taste, every thing unfinished and dirty, grand plans for the future, nothing pleasant nor even comfortable at present".

[7] A number of the present furnishings reflect Castlereagh's career, including a portrait of the French emperor,[8] and chairs elaborately embroidered for the delegates who redrew the map of Europe at Vienna.

[9] During the three-day "Year of Liberty"[10] in Ards and north Down, 10 to 13 June 1798,[11] Mount Stewart was briefly occupied by the United Irish insurgents.

Porter caricatured the master of Mount Stewart as Lord Mountmumble, an inarticulate tyrant who has a dog shot for the temerity of barking.

He married Lady Frances Anne Vane-Tempest, the greatest heiress of her time, in appreciation of which he styled himself Robert Vane and ordered a further enlargement of the house replacing what remained of its 18th century fabric.

Controversially in 1847, while spending £15,000 on the refurbishment, the Marquess of Londonderry gave just £30 to local soup kitchens for famine relief,[21][22] and as the hunger persisted rejected rent reductions.

Even as she embarked upon of the construction a castellated summer residence(Garron Tower), the Marchioness not only reduced the rents of her tenants, but in dire cases of potato blight, waived them altogether.

George Henry Robert Charles William Vane-Tempest, 5th Marquess of Londonderry lived at his wife's ancestral property, Plas Machynlleth in Wales.

Meeting the unionist leader Sir Edward Carson at a luncheon at Bad Homburg in August 1913, Wilhelm II remarked that having seen a photograph of the gardens, he believed that they must be very beautiful.

Through the gardens of Mount Stewart the Kaiser had been probing intelligence that in the event of a European war conflict in Ireland might stay Britain's hand.

[32] In 1921, Charles Vane-Tempest-Stewart, 7th Marquess of Londonderry, (1878–1949) accepted office as Minister of Education in the unexpected fruit of unionist agitation, the new home-rule Parliament of Northern Ireland.

[33][34][35] At Mount Stewart it was a suspicion Londonderry appeared to confirm when, following on a visit to Hitler in Berlin, in May 1936 he entertained the German Ambassador to London, Joachim von Ribbentrop.

[48] In 2015, the National Trust completed an extensive restoration of the house and its contents as well as the purchase of 900 acres (360 hectares) of the wider estate, thus re-uniting it, and plan[needs update] to open for visitor access.

The National Trust refurbishment, completed in 2015, sought to restore the interiors to how they appeared in the 1950s when the house belonged to Lady Edith, the seventh Marchioness.

The area is frost-free and, as Lady Edith discovered, Mount Stewart enjoys island conditions, the atmosphere is humid and, in hot weather, there are heavy dews at night.