National Trust Ranger Toby Edwards was recently clearing rhododendron bushes on the demesne when he noticed markings on one of the trees.
They appear to have been made by soldiers and airmen stationed at the estate during the Second World War. One of the carvings looks to have been by a Royal Air Force airman in 1940.
Close by, another inscription reads, 'Victory Hours R.A.F.' A third tree inscribed with initials also appears to have been claimed by the soldiers.
Toby said that he had seen carvings on the trees in other parts of the grounds, but added that none had been as well preserved as the ones that he discovered last month.
Records show that the Headquarters Company of the Royal Engineers was based at Mount Stewart, with Officer Commanding Major Fulton in the house and junior ranks billeted in the buildings around the coach house and stables.
Slabs of concrete where bunkers and outpost service buildings are believed to have once stood were also uncovered. The buildings are thought to have been used as house showers, toilets, storage, and Nissen huts during the Second World War.
During the renovation work, similar buildings were also found buried in the woods at the back of the estate.
Research shows that attic rooms connected to the estate's house were used as a convalescence ward in both World Wars.