One of the themes of Memento Mori by Muriel Spark is how people deal with mortality. When the characters receive calls from unknown callers saying that they must remember that they will die, the characters react in different ways. For example, Dame Lettie Colston reacts with anger and fear. However, Charmian (Dame Lettie's senile sister-in-law) reacts merely with acquiescence when she receives the call. She has forgotten much about her life, but she recalls the realities of life—that death impends for all (she also, strangely, remembers everything about the books she has written). Mrs. Pettigrew, the housekeeper, is too busy to think of death. When she receives a call reminding her that she will die, she simply decides to forget about it. She drops the thought from her mind. Alec Warner, a sociologist, spends his time collecting notes on the elderly, but his notes are eventually lost. This is his method of coping with mortality.

A related theme is that one can only die gracefully if one accepts the will of God. One of the few characters who accepts death with resignation is Jean Taylor, who is in a nursing home where she receives humiliating treatment. Jean says to herself, "If this is God's will then it is mine." From this resignation, she regains her dignity—even in the face of the degrading treatment she receives at her nursing home, where she and the other women are referred to as "grannies."

A final theme in the book is the way those who know death is near fritter away their time and do not take full advantage of life. Early in the book, the contemplative Jean Taylor muses on the way the elderly spend their time. Some fight about their wills, while others think of their youth. Dame Lettie Colston spends her time being cruel to others while Charmian has entirely lost her memory, save for her books. Jean, in contrast to many of the other characters, faces her death with good grace and resignation. The others spend their waning days without realizing that they should give up their petty and mean concerns.