The inevitability of death serves as the central theme in On the Beach. As the radiation cloud approaches Melbourne, the characters in the novel become intensely aware of their impending end. They are all relatively young and struggle with the profound injustice of their circumstances, much like Peter does when he tries to explain it to his wife:

"It's the end of everything for all of us," [Peter] said. "We're going to lose most of the years of life we've looked forward to, and Jennifer's going to lose all of them."

Shute explores how different individuals cope with the harsh reality of death. Some characters in On the Beach remain in denial, choosing to disregard their impending doom. For example, Mary plans a garden she will never see bloom, and Towers buys sentimental gifts for his deceased family, maintaining his loyalty to his late wife.

Conversely, some characters turn to indulgence as a way to deal with death. Sir Douglas Froude decides to drink all the wine at the Pastoral Club. At the beginning of the novel, Moira is described as a somewhat promiscuous drinker. However, her growing feelings for Towers transform her into a selfless heroine; she cuts back on her drinking and self-pity to care for him, even though she knows he will never reciprocate her affections. This selflessness is depicted as a noble and commendable response to mortality.

Another reaction to death is embracing daring challenges. In the story, John Osborne's love for racing exemplifies this approach. Osborne is a rather dull and timid man until he gets behind the wheel of his Ferrari. When Towers asks about the car’s performance in a race, Osborne's description is similar to recounting a thrilling roller-coaster experience:

"You get scared stiff. Then directly it's over you want to go on and do it again."

On the Beach serves as a cautionary narrative, designed to warn readers about the dangers of nuclear warfare. Written during the height of the Cold War arms race between the United States and the Soviet Union, there was widespread fear that World War III was inevitable.

Shute emphasizes the characters' responses to the aftermath of the war rather than the conflict itself, yet he clearly views the idea of a "winnable" nuclear war as both absurd and dangerous. This perspective is echoed in the concluding conversation between Peter and Mary:

"Couldn't anyone have stopped it?" [Mary asked]

"I don't know. Some kinds of silliness you just can't stop," [Peter] replied. "I mean, if a couple of hundred million people all decide that their national honor requires them to drop cobalt bombs on their neighbor, well, there's not much that you or I can do about it. The only possible hope would have been to educate them out of their silliness."

In the novel, Shute emphasizes the enduring human ability to maintain hope, even in the bleakest of circumstances. The Royal Australian Navy sends the U.S.S. Scorpion on two missions, hoping to find any signs of life. Meanwhile, Peter and Mary focus on planning a garden. However, the most significant example of hope in the narrative is Towers's strong belief that he will be reunited with his wife and children in the afterlife. His hope serves as an inspiration for Moira by the end of the novel.

She uncorked the bottle. It was ten minutes past ten. She earnestly said, "Dwight, if you’re already on your way, wait for me."
Then she placed the tablets in her mouth and washed them down with a sip of brandy, sitting behind the wheel of her large car.

Euthanasia, often referred to as mercy-killing, is a contentious topic in On the Beach. Within the narrative, euthanasia is used as a means to prevent people from enduring the agonizing effects of radiation poisoning.

Radiation sickness is known to severely incapacitate the human body. To combat this, the Australian government supplies a free drug to the populace, enabling them to end their lives peacefully. Additionally, syringes are provided so that individuals can receive an injection from someone else. Mr. Goldie, the pharmacist, is acutely aware of the gravity of this situation.

"There must be terrible complications over the religious side," he remarked. "I suppose then that it's a matter for the individual."

Indeed, there are "terrible complications." One of the novel's most harrowing moments unfolds when Peter tries to explain to Mary that she may have to end their infant daughter's life to spare her the agony. Mary is understandably horrified and accuses Peter of wanting to leave her and the baby for another woman. In frustration, Peter describes a grim scenario to make her grasp the situation:

"There's something else you should consider," he said. "Jennifer might outlive you... You can struggle on as long as you can endure until you die. But Jennifer might still be alive. She could survive for days, crying and vomiting in her crib, lying in her own mess, with you dead on the floor next to her and no one to help her. Eventually, of course, she'll die. Do you want her to die like that? If you do, I don't." He turned away. "Think it over, and don't be such a bloody fool."

Eventually, Peter assumes the responsibility of administering the drug to his daughter. Shute tackles this unsettling topic with a straightforward approach. The author seems less preoccupied with the ethical debate surrounding euthanasia and more focused on the dire circumstances that might compel a father to take his own daughter's life.

Shute's writing is heavily shaped by his moral beliefs and perspective on life. In his novels, for example, a married man never willingly participates in an affair. In A Town Like Alice (1950), the main character, Jean Paget, interrupts a passionate moment with her lover just in time to maintain her virginity before they marry, which soon occurs in the narrative. The characters in Shute's stories are usually people who have a strong sense of self, understand their duties, and meet them despite significant obstacles—Jean Paget constructs a town; the Corbetts survive the Blitz; and Australia maintains its routine even amid nuclear devastation.

The novel revolves around the themes of sensibility, order, and control. As a result, there are no riots or disorderly conduct, with the minor exception of Moira. Uncommon occurrences, such as the peculiar final Grand Prix, are dealt with pragmatically. The crew of the Scorpion largely keeps their cool, and as the unavoidable conclusion draws near, individuals quietly retreat, much like the shops, each preparing for the end in their own way.