The Bowes family had held the house since the 15th century. By the 19th century it was one of three principal seats belonging to the Earls of Strathmore and Kinghorne - alongside Glamis in Forfarshire and Gibside near Gateshead. For decades after 1820 the English and Scottish branches were separated by the unusual legal status of John Bowes, the eldest son of the 10th Earl. John was illegitimate under English law because his parents married after he was born, and illegitimate under Scottish law because they had no Scottish domicile - so the Scottish earldom passed past him, but he inherited a life interest in the English wealth. He poured it into Streatlam, into the Bowes Museum he founded down the road in Barnard Castle, and into a racing stable that produced four Derby winners. He died without legitimate issue in 1885, and the estate reunited with the earldom - by then four generations removed from anyone who actually lived there.

Patrick Bowes-Lyon, Lord Glamis - the future 15th Earl - had been living at Streatlam since at least 1915. The 14th Earl decided to sell. In 1922 the Daily Telegraph reported the castle and 1,198 surrounding acres on offer for £37,500, with an option on a further 4,806 acres. The bulk went to private tenants. The remainder fetched £100,000 at auction. Lord Glamis moved to a substantial farm near East Grinstead and stayed there until he succeeded his father in 1944. There was no financial pressure. The estate's coal income still flowed, and the Earl still owned Wemmergill, St Paul's Walden Bury in Hertfordshire, and Gibside. The reason was cultural. The Russian Revolution had concentrated minds about ostentation. The First World War had emptied the servants' hall - finding the staff to run a 24-bedroom house was increasingly impossible. And, frankly, the family preferred Glamis. Lady Strathmore, though ill, hurried to Streatlam to rescue what she could; many pieces went north to Glamis. The armorial ceilings John Bowes had installed were carefully removed and taken to the Bowes Museum, where they still hang.

After the Second World War, the great houses of Britain came down by the hundreds. Heating bills, taxes, vanished domestic labour, repair bills no rent roll could meet - country house after country house was abandoned, sold, or simply destroyed. Streatlam joined the queue. In 1959 the Territorial Army arrived with explosives and used the shell as a training exercise. The walls came down in a methodical sequence; the gardens grew over what remained. Today only Streatlam Park and the entrance lodges still stand. In November 2017 the Bowes Museum opened an exhibition on the castle's history - paintings that once hung on its walls, scale models of the house and the surrounding estate. The exhibition travelled to Glamis the following March, where Lady Strathmore's salvaged pieces had ended up nearly a century before. The house exists now only as memory, model, and lodge gate.