While living in London, Hobson was exposed to the Social Democrats and H.M. Hyndman, Christian Socialists, and Henry George's Single-tax system.

First described by Mummery and Hobson in the book Physiology of Industry (1889), underconsumption was a scathing criticism of Say's law and classical economics' emphasis on thrift.

During his coverage of the Second Boer War, Hobson began to form the idea that imperialism was the direct result of the expanding forces of modern capitalism.

Imperialism gained Hobson an international reputation, and influenced such notable thinkers as Vladimir Lenin and Leon Trotsky, and Hannah Arendt's The Origins of Totalitarianism (1951).

R. H. Tawney wrote the following in The Acquisitive Society (1920): The greater part of modern property has been attenuated to a pecuniary lien or bond on the product of industry which carries with it a right to payment, but which is normally valued precisely because it relieves the owner from any obligation to perform a positive or constructive function.

It is questionable, however, whether economists shall call it "Property" at all, and not rather, as Mr. Hobson has suggested, "Improperty," since it is not identical with the rights which secure the owner the produce of his toil, but is opposite of them.

Historians Peter Duignan and Lewis H. Gann argue that Hobson had an enormous influence in the early 20th century among people all around the world: Hobson's ideas were not entirely original; however his hatred of moneyed men and monopolies, his loathing of secret compacts and public bluster, fused all existing indictments of imperialism into one coherent system....His ideas influenced German nationalist opponents of the British Empire as well as French Anglophobes and Marxists; they colored the thoughts of American liberals and isolationist critics of colonialism.

Notably, John Gallagher and Ronald Robinson in their 1953 article The Imperialism of Free Trade argued that Hobson placed too much emphasis on the role of formal empire and directly ruled colonial possessions, not taking into account the significance of trading power, political influence and informal imperialism.

They also argued that the difference in British foreign policy that Hobson observed between the mid-19th-century indifference to empire that accompanied free market economics, and the later intense imperialism after 1870, was not real.

In Imperialism he advocated their "gradual elimination" by an international organization: "A rational stirpiculture in the wide social interest might, however, require a repression of the spread of degenerate or unprogressive races".

[11][12] While it can be said the 1902 work reflected the Social Darwinism trend of the time, Hobson left this section mainly unchanged when he published the third edition in 1938.

[20] Following Hobson's January 1900 article Capitalism and Imperialism in South Africa, Labour leader Keir Hardie in February 1900 repeated the same message in paraphrased form accusing "half a dozen financial houses, many of them Jewish" of leading the UK to war.

[21] However, as the British working class tended to support the war in South Africa, Hobson's zeal in attacking "Jew Power" in South Africa and manipulation by a secret "racial confederacy" failed to attract popular support in Britain, though "anti-Alien" sentiments continued to be an issue.

Among commentators in Continental Europe, in particular France and Germany, the alleged "robbery committed by international Jewry" was frequently linked by right-wing antisemites to "British imperialist piracy" during the Second Boer War.