Initially, the novel fared poorly in both the United Kingdom and the United States, selling only 3,076 copies between 1907 and 1914. The book fared slightly better in Britain, yet no more than 6,500 copies were pressed before 1914. Although sales increased after 1914, the novel never sold more than "modestly" throughout Conrad's lifetime. The novel was released to favourable reviews, with most agreeing with the view of The Times Literary Supplement, that the novel "increase[d] Mr. Conrad's reputation, already of the highest." However, there were detractors, who largely disagreed with the novel's "unpleasant characters and subject". Country Life magazine called the story "indecent", whilst also criticising Conrad's "often dense and elliptical style".
In modern times, The Secret Agent is considered to be one of Conrad's finest novels. The Independent calls it "[o]ne of Conrad's great city novels" whilst The New York Times insists that it is "the most brilliant novelistic study of terrorism". It is considered to be a "prescient" view of the 20th century, foretelling the rise of terrorism, anarchism, and the augmentation of secret societies, such as MI5. The novel is on reading lists for both secondary school pupils and university undergraduates.