Because Abe Lincoln Grows Up is biographical rather than fictional, it lacks a true plot. There is no rising action leading to a climax that changes the lives of the people involved. Instead, Sandburg creates a realistic background—the frontier wilderness of Kentucky, Indiana, and Illinois—and then populates it with historical figures.

The major characters are Abe Lincoln; his mother, Nancy Hanks Lincoln; and his stepmother, Sarah Bush Johnston Lincoln.

Sandburg characterizes the young Abe Lincoln as mischievous with his friends and serious in his work and studies.

Lincoln's developing sense of responsibility and his growing interest in the law, government, and the world beyond the wilderness receive a good deal of attention from his biographer. His work ferrying passengers out to steamboats on the Ohio River and his trip to New Orleans on a flatboat demonstrate the future president's independence and his resourcefulness. Sandburg's emphasis on Lincoln's positive personality traits develops one of the biography's major themes: success is often the result of perseverance in the face of obstacles.

The biography covers Lincoln's relationships with family members as well. A colorful and interesting character, Lincoln's father, Tom, is devoted to his family and does not drink much or curse. But he clings to the old ways, as reflected in his impatience with education, which he believes should be limited to "readin', writin', and cipherin'." He does not understand his son's need for more knowledge, but when Abe's stepmother interferes, Tom yields to her.

Thus Sandburg presents the timehonored theme of the old giving way to the new.

Abe's mother and stepmother are depicted with warmth and sympathy.

Both love and nurture their children, although Nancy Hanks's origins are somewhat mysterious, and she is described as a bit "strange." Sarah Bush Johnston, on the other hand, is outgoing, drawing the two motherless Lincoln children to her as if they were her own. When she holds him to her, Abe feels "like a cold chick warming under the soft feathers of a big wing." She embodies the theme of overcoming hardships with love.

The many rather vague and sketchy minor characters are generally developed as much as necessary in the context of the biography. Sandburg introduces characters who actually play no direct role in the Lincoln story but who add depth and richness. Information about the activities of Napoleon Bonaparte and Andrew Jackson lends historical perspective to the work, while the description of Mike Fink, a character of tall tales, provides a glimpse of the myths inspired by the frontier.