Old Mortality is focused on questions of death and legacy and what the legacies that we inherit mean for our processes of self-discovery. Miranda is fascinated by photographs, keepsakes, and stories that give her windows into what her various relatives were and are like—Cousin Eva, Uncle Gabriel, and her late Aunt Amy especially.

Miranda's Uncle Gabriel sends letters and poetry that talk about Aunt Amy's death as if it were a glorious event that freed her from the chains of "Old Mortality." Aunt Amy also seems to have subscribed to an exaggerated, romantic view of things. Within the span of a week, Aunt Amy goes from a wild-partying young girl to complaining that she is old and chained down by her marriage to Uncle Gabriel, sentiments she recorded shortly before her death. Over and over, Miranda sees that her family members are not as beautiful as they have been described, not as pious as they are supposed, and so on.

This realization of the difference between how her family appears in photographs and stories versus how they are in reality leads to Miranda's disillusionment with her ideas about good and evil. Ultimately, she must abandon her once-clear sense of how things should be, which had been part of her childhood naivety. By the end of the novel, she is left with more questions than answers.

The process of growing up and beginning to understand the difference between stories and reality can easily put someone into conflict with their family and community. Miranda gains a greater understanding of her family and the ways in which the stories she's been raised on are not entirely accurate. As this happens, she's pushed to figure out her own position within her family. She starts to consider this as part of an open process, rather than treating the continuation of her family legacy as a given necessity. Death continues to be the background for much of this process—the ultimate assurance that change will always happen and that clean, simple answers cannot hold up for long.