Muir made a key contribution to the development of Liberal political thought in the 1920s and 30s. His book Liberalism and Industry strongly influenced leading Liberal politicians including Lloyd George. In 1921 Muir was one of the co-founders, along with John Maynard Keynes, of the Liberal Summer School which was a major source of ideas for the party between the wars. From 1923 to 1926 he was editor of the Weekly Westminster. In the 1930s he was one of the key figures in the organisation of the Liberal Party, serving as Chairman of the National Liberal Federation 1931-33 and its President 1933-36. As President, he wrote much of the NLF’s The Liberal Way (1934) which was a strong statement of Liberal policy and of the principles underlying modern social Liberalism.