Fillmore married Caroline Carmichael McIntosh in 1858, a wealthy 44-year-old widow. She insisted on a prenuptial agreement, Due to her significant fortune inherited from her first husband, Ezekiel C. McIntosh. They honeymooned in Europe, returned to New York and purchased a mansion on Niagara Square in Buffalo.
The Fillmores' 16 years of marriage was spent in the Niagara Square mansion.