To increase his income to support his family, from 1893 Morel began writing articles against French protectionism, which was damaging his company's business. He came to be critical of the Foreign Office for not supporting Africa and African decolonisation movements. His vision of Africa was influenced by the books of Mary Kingsley, an English traveller and writer, which showed sympathy for African peoples and a respect for different cultures that was very rare at the time.
Elder Dempster had a shipping contract with the Congo Free State for the connection between Antwerp and Boma. Groups such the Aborigines' Protection Society had already begun a campaign against alleged atrocities in Congo. Due to his knowledge of French, Morel was often sent to Belgium, where he was able to view the internal accounts of the Congo Free State. The knowledge that the ships leaving Belgium for the Congo carried only guns, chains, ordnance and explosives, but no commercial goods, while ships arriving from the colony came back full of valuable products such as raw rubber and ivory, led him to the opinion that Belgium's policy was inhumane. According to the Belgian Prof. Daniël Vangroenweghe, King Leopold II gained 125 million present day euros from the exploitation of the Congolese people, mainly from rubber. Other Belgian sources calculated that the profits from the Congolese exploitation prior to 1905 were some 500 million present day euros.
The gains from the exploitation of rubber through the state and other companies like the Anglo-Belgian India Rubber Company (ABIR) were huge. The original value of the ABIR shares was 500 francs (1892 gold francs). In 1903 the shares had risen to 15,000 gold francs. The company felt obliged to let the Bourgeoisie share profits with the upper class. The dividend in 1892 was 1 franc. In 1903 the dividend was 1,200 francs, more than the double of the original price of a share. These enormous gains came from horrible exploitation, and the equator region became a green hell. The scope of the destruction, together with disease and famine from forced labor, killed half of the population of the colony.
In 1900, Morel began his campaign with a series of articles in the weekly magazine Speaker. He realized that King Léopold II of Belgium, the absolute monarch of the Congo Free State, had created a forced labour system of huge dimensions, in effect slave labour. Despite having risen to be Elder Dempster's Head of trade with Congo, Morel resigned in 1902 to further his campaign. Morel became a full-time journalist, first finding a job in the editing of a recently founded periodical, East Africa, then founding in 1903 his own magazine, the West African Mail, with the collaboration of John Holt, a businessman and a friend of Mary Kingsley who feared the application of the Congo Free State system upon the rest of the West Africa. The Mail was an "illustrated weekly journal founded to meet the rapidly growing interest in west and central African questions" (Hochschild 2004, p.186). In this period Morel published several pamphlets and his first book, Affairs of West Africa.
In 1903 the British House of Commons passed a resolution on the Congo. Subsequently the British consul in Congo, Roger Casement, was sent for an inspection. His 1904 report, which confirmed Morel's accusations, had a considerable impact on public opinion. Morel met Casement just before the publication of the report and realized that he had found the ally he had sought. Casement convinced Morel to establish an organisation for dealing specifically with the Congo question, the Congo Reform Association. Branches of the Congo Reform Association were established as far away as the United States.
The Congo Reform Association achieved the support of famous writers such as Joseph Conrad (whose Heart of Darkness was inspired by a voyage to the Belgian Congo), Anatole France, Arthur Conan Doyle and Mark Twain. Conan Doyle wrote The Crime of the Congo in 1908, while Mark Twain gave the most famous contribution with King Leopold's Soliloquy. Morel's best allies were perhaps missionaries who furnished him with eyewitness accounts and photographs of the atrocities, such as the Americans George Washington Williams and William Henry Sheppard and the British John Harris and Alice Harris. The chocolate millionaire William Cadbury, a Quaker, was one of his main financial backers. The American civil-rights activist Booker T. Washington participated in the campaign. The French journalist Pierre Mille wrote a book with Morel, while the Belgian socialist leader Emile Vandervelde sent him copies of Belgian parliamentary debates. Morel had secret connections with some agents of the Congo Free State. Even the Church of England and American religious groups backed him.
In 1905 the movement won a victory when a Commission of Enquiry, instituted (under external pressure) by King Léopold himself, substantially confirmed the accusations made about the colonial administration. In 1908 the Congo was annexed to Belgium and put under the sovereignty of the Belgian Parliament. Despite this, Morel refused to declare an end to the campaign until 1912 because he wanted to see actual changes in the situation of the country. The Congo Reform Association eventually ended operations after 1911.
The Union of Democratic Control became the most important of all the anti-war organizations in Britain, with membership reaching 650,000 by 1917. His political courage was praised by people as Bertrand Russell and the writer Romain Rolland, but his leading role in the pacifist movement exposed him to violent attacks led by the pro-war press. He was pictured as an agent of Germany in the Daily Express, a newspaper that also listed details of future UDC meetings and encouraged its readers to attend and break them up. The accusation gained some credibility when Roger Casement, who was known as a friend and supporter of Morel, was hanged for treason (he had contacted the Germans seeking support for Irish nationalism). Morel was the occasional victim of physical assaults.'
Morel was severely critical of the Treaty of Versailles, warning that it would lead to another war. He did not give up his career as a journalist, becoming director of the magazine Foreign Affairs which became the most authoritative voice of English left about foreign politics.
In April 1918, he joined the Independent Labour Party, and began to feed his views into the Labour Party to which it was affiliated and which adopted his critical view of the Treaty of Versailles. Morel explained his decision to join the Independent Labour Party to a friend:
With his foreign affairs specialty, he was expected to be appointed as Foreign Office minister during the government of Ramsay MacDonald in 1924, but MacDonald decided to serve as his own Foreign Secretary. MacDonald led an attempt to buy Morel off by gathering together a large number of senior Labour politicians to nominate him for the Nobel Prize for Peace, but it did not prevent Morel from remaining a forceful critic of MacDonald's foreign policy. In August 1924 he is believed to have persuaded MacDonald to recognize the communist government in the Soviet Union and nominations on the Anglo-Soviet trade treaty.
Shortly after his re-election in the 1924 general election, Morel suffered a fatal heart attack at the farm near Bovey Tracey, Devon where he lived.