Known as "Marmee" by her daughters, Mary serves as the family's moral anchor. She volunteers at Newcord's local church, aiding families of Union soldiers and encouraging her daughters in charitable acts. Her compassion is evident when she shelters John, a runaway slave forcibly conscripted by the Confederates. After John takes Beth hostage in panic, Mary negotiates his surrender, immediately forgives him, and offers sanctuary despite the legal risks.

Mary's relationship with Aunt March is complex. Upon arrival in Newcord, the family faces Aunt March's initial coldness due to resentment over Frederic's lack of contact. Mary expresses understanding of this bitterness, acknowledging the unintentional hurt from Frederic's absence. Despite financial hardship, Mary ensures the family maintains dignity and self-sufficiency. Meg and Jo take jobs to support household expenses while Mary sustains their emotional resilience during Frederic's military service.

Mary remains a stabilizing presence throughout the series. She guides her daughters through personal challenges, including Jo's conflicts with Amy and Beth's health struggles. Her leadership is demonstrated when she travels to Washington, D.C., to nurse the severely ill Frederic back to health, leaving the sisters under Hannah's temporary care. The series concludes with the family reunited after Frederic returns from the war, their bonds strengthened by shared adversity.