Hicks was the daughter of William J. Day and Anna (née McCarron) Day. Hicks described her father, a lawyer and an influential judge in Boston, as her "greatest influence". The child of poor Irish immigrants, William Day became one of the wealthiest men in South Boston as a result of his law practice, real estate investments and his role as director of South Boston's Mount Washington Cooperative Bank. Day was admired by Boston's Irish community: as a banker he provided assistance to families struggling to make mortgage payments and as a judge he was particularly lenient towards juvenile defendants. In her own political career, Hicks would benefit from her father's reputation.