During the First World War, he served as Director of Restriction of Enemy Supplies.

In 1922, Mitchell-Thomson was Parliamentary Secretary to the Board of Trade and from 1924 until 1929, he served as Postmaster General.

In 1932, Mitchell-Thomson resigned from the House of Commons and was raised to the peerage as Baron Selsdon, of Croydon in the County of Surrey.

The results of the Selsdon Report were issued as a single Government White Paper in January of the following year.

[1] Lord Selsdon died at his home in 20 Grosvenor Square, London, in December 1938, aged 61, and was cremated at Golders Green Crematorium, his ashes later buried in Edinburgh.