The social and political changes of the last century are reflected in the fate of Osterley.

After the 7th Earl of Jersey died in 1915, Osterley was used only for summer parties. The 9th Earl was 13 when he succeeded his father in 1923.

His mother, recently remarried, set about introducing a little modernity to Osterley, electric lights and central heating. Lady Slessor, as she became, brought in the Robert Adam expert A.T. Boulton, who worked to preserve the vaulted Servants' Hall on the ground floor and oversaw the re-gilding of some of the furniture.

Osterley, blessed with the finest work of Adam and with modern conveniences, was advertised for let in Country Life in 1926. No one suitable came forward. The house continued to be used for parties until, in 1937, the Jerseys moved in for 18 months. The 9th Earl concentrated mainly on Osterley's garden, lightening the mood by replacing conifers with flowers.

It became one of the first stately homes to open to the public, at Easter 1939. Lord Jersey established displays of contemporary art on the upper floors. When war broke out, the grounds were used by the Home Guard for training, making Osterley a target. Several bombs were dropped in the grounds; fortunately they missed the house.

After the war, Lord Jersey sought to find a permanent solution to the future of Osterley. In 1949, he gave the house and the central core of the landscaped park to the National Trust.

Without an endowment, raising funds for the upkeep of Osterley posed serious problems for the Trust. It was decided to lease the house and park to the Ministry of Works (now the Department of the Environment) for 99 years, with the Royal Parks maintaining the grounds and the Victoria and Albert Museum administering the house. It was re-opened to the public in 1953.

In 1989, the DOE surrendered its lease and management passed back to the National Trust.

In 1996, the Trust purchased 153 acres of Osterley Park Farm which formed part of the original park at Osterley. This acquisition was made possible by a grant from the Heritage Lottery Fund.

The entire estate is being restored to its 18th-century form. Under one management, it can be maintained in its original splendour.