Conrad wrote in his author's note to Heart of Darkness that the novel was "authentic in fundamentals" and that it represented "experience pushed a little (and only a very little) beyond the actual facts of the case." In fact, many details and even characters in Heart of Darkness come from real life and also from some pages of Conrad's Congo diary.
Conrad started his career as a sailor on a French ship. In 1878 when he was at the age of 20 he worked on an English steamer. For the next 16 years he sailed under the flag of Britain, and became a British subject in 1886. Life in the merchant marine took him to ports in Asia and the South Pacific, where he gathered material for the novels he still did not know he was going to write.
Many of the characters in Heart of Darkness are based on real-life personages. It can be argued that Marlow is Conrad. It is trute true that Marlow was born in England, not Poland, and he never gave up sailing to write; yet if we ignore these two facts, the differences between the two men are not striking. For example, Marlow's sarcastic and painful comments on the atrocities committed in the Congo are paraphrases of Conrad's own responses to what he had witnessed in the Congo and registered in his diary: "Everything is repellent to me here," he wrote from the Congo, "Men and things, but especially men." The "scramble for loot" disgusted him; the maltreatment of the black Africans sickened him. After Conrad's experiences in Africa, he returned to England traumatized, just like Marlow after his coming back to England. In the novel, and after Kurtz's death Marlow catches a fever that very nearly kills him. This is also partly autobiographical: Conrad came down with dysentery during his voyage on the Congo.
Likewise, Marlow's aunt is based, at least in part, on Marguerite Poradowska, who was related to Conrad by marriage. She was not a true aunt, but he addressed her that