The characters of Mary Carson and Fiona Cleary in The Thorn Birds are similar in numerous ways, but the differences between them become more significant in shaping the plot. Both Mary and Fiona are white Australians. Both women marry for convenience and security rather than for love. A major point of contrast concerns their relative status. Mary is a wealthy woman who dominates the much-poorer Fiona and her husband. Overall, Mary is manipulative and driven by her lust for power. Fiona is a mother who is motivated by her varying degrees of love for her different children.

While she was married, Mary became the dominant partner over her husband. As a widow, she has considerable clout in the region around her estate. Mary uses her love for Ralph and her desire for power to advance his career. While she is ostensibly a devout Catholic, she is more concerned with influencing the church by maneuvering for Ralph’s advancement in the hierarchy. Her obsession with power extends to her domination over Fiona, Paddy, and their children.

Fiona’s personality revolves around her involvement with her children, primarily her favoritism to Frank. As Paddy’s wife, she works the land and, by moving to Drogheda, places herself under the control of Mary. Her influence in the novel overall is less pronounced than that of Mary, but her neglect of Meggie sets in motion the events that lead to her daughter’s involvement with Ralph.