Elliot was elected Conservative MP for Lanark in the 1918 General Election, lost his seat in 1923 but returned as MP for Glasgow Kelvingrove in 1924. He was Parliamentary Under-Secretary for Health in Scotland (1924��"26) and joined the Cabinet in 1932 as Minister of Agriculture, later becoming Secretary of State for Scotland (1936-38) and Minister of Health (1938-40).

With experience in both the Ministries of Agriculture and Health, he was well placed to argue that controlled milk production would help both the recession and child malnutrition. As chair of the Empire Marketing Board, he made funds available for research into the marketing of dairy products, including the feeding of milk to school children in Scotland. This research was carried out by John Boyd Orr and his research team at the Rowett Research Institute (1926-28).

John Boyd Orr

"the only conclusion possible is that they [the results] have a wide public health significance, especially with the nutrition of school-children."

In November 1929, he introduced the Education (Scotland) Bill into the House of Commons as a private member, with a view to allowing Scottish Local Education Authorities to provide free or subsidized school milk. The costs involved in the scheme meant that it was not a success.

Elliot was also behind the Milk Act 1934 which established a 'Milk in Schools' scheme run by the Milk Marketing Board, with the aim of supplying a third of a pint of milk daily to 50 per cent of elementary school children, either at half cost or free. This was achieved by 1939.