The themes of duty and responsibility are intricately interwoven throughout the book, manifesting on various levels. On a national scale, the narrative underscores the British dedication to duty and efficiency, which enabled the methodical colonization worldwide. This theme is echoed in the Belgian colonization of the Congo, which serves as the story's central focus. On a personal level, Conrad delves into these themes through Marlow's position as a captain, where he is responsible for his crew and bound by his obligations as the boat's commander. These themes also extend to the different individuals Marlow encounters on his journey.

In Heart of Darkness, duty and responsibility frequently center around the manner in which one performs their tasks. Completing a job well earns respect; simply fulfilling one's duties is regarded as honorable. However, Conrad refrains from idealizing the worker. Often, workers are depicted engaging in pointless tasks, such as the Africans blasting away at a rock face to construct a railway, even though the rock remains unchanged and the cliff poses no barrier. The Company's Manager, supposedly responsible for running the operation efficiently, fails to maintain order and, although obeyed, is not respected. In contrast, the Foreman gains Marlow's admiration for his diligent efforts, notably tying up his waist-length beard to crawl in the mud beneath the steamboat—a practical yet seemingly irrational act in the jungle setting.

The first section of the novel concludes with Marlow contemplating how Kurtz would manage his work. There is a broader context where the themes of work and responsibility hold significance. Marlow remarks, ‘‘I don't like work—no man does—but I like what is in the work—the chance to find yourself.’’ Through Kurtz's actions in Africa, his moral decline is revealed. For Marlow, a self-imposed obligation to remain loyal to Kurtz ultimately leads him to lie to Kurtz's fiancée.