In 1939 or 1940 the castle was requisitioned for the war effort and used for tank development. It was returned to the family not before 1954. The family could not afford use or upkeep and offered to give it away to the National Trust and other institutions, however, in those depressed post-war years they did not find takers. In order not to have to pay taxes they stripped the castle and removed the roof.
In 2000 the Lowther Estate and English Heritage jointly commissioned a team of historians, landscapers, architects and engineers to review the status of the castle and its grounds, and they produced the Lowther Castle & Garden Conservation Plan. In 2005 the estate formed an informal partnership with the Northwest Development Agency, English Heritage, Cumbria Vision and the Royal Horticultural Society to regenerate the site. The objectives were to consolidate the ruin, restore the 50-acre (200,000 m2) garden and open the site to the public. The castle and 130 acres of grounds were transferred to a charity called the Lowther Castle and Gardens Trust in 2007 and the site opened to the public on 22 April 2011.
