Judge, Philadelphia, approximately 1780-1831

Around 1800, Costin moved from Mount Vernon to Washington City, what later became known as Washington, D.C. About that time, he married Philadelphia "Delphy" Judge, whom Martha Washington had given to her granddaughter Elizabeth Parke Custis Law as a wedding present in 1796. She and her children were manumitted in 1807 by Thomas Law, Elizabeth's husband. (see below).

In 1812, Costin built a house on A Street South on Capitol Hill. There he and his wife Delphy raised a large family.[7][8][9]

Ona likely never again saw her Mount Vernon family. Her mother, Betty, died in January 1795. In 1802, her younger sister Delphy was inherited by Eliza Parke Custis Law, the fate that Ona had fled to avoid.