Exile and isolation are major themes in Mary Shelley's The Last Man. The most obvious physical example of these themes is Lionel Verney's being the last surviving human being at the end of the story. He slowly watches everyone he loves and the other humans around him die off as the plague takes civilization down. In the end, he is alone with only books and the splendor of nature to comfort him.
However, exile was part and parcel in Lionel's life from the start. He and his sister Perdita are noble-born, but their father fell into disgrace and they were not allowed to be part of the royal court. They lived poor, even savagely, in exile, until a friendship with the prince Adrian civilizes the angry Lionel and brings him back into society. The rest of the novel plays out as a series of exiles. Lionel loses his family, then society, then is utterly alone.
These themes have a connection with Shelley's real life. They reflect her growing feelings of loneliness in the wake of the death of her child and husband, as well as the loss of their fellow writer friends. For her, it felt as if a whole movement—the Romantic movement, to be exact—were ending.