In 1796 Law married Elizabeth Parke Custis (d. 1832), the eldest granddaughter of Martha Washington and step-granddaughter of George Washington.
Soon after their marriage, Law and Elizabeth moved temporarily into a new house built on 6th Street SW as part of development in that area.
The Federal-style house became known as "Honeymoon House"; the Laws lived there from about March through August 1796.
They next moved to the house they had built for them.

Due to differences, Law and his wife separated in 1804, and they were divorced on 18 January 1811.
The couple had one daughter who survived infancy, Elizabeth Law (19 January 1797 – 9 August 1822).
She married Nicholas Lloyd Rogers (known as Lloyd N. Rogers) of Druid Hill, Baltimore on 5 April 1817.

When Law and his wife, known as Eliza P. C. Law, separated, he made a deed of certain real estate as jointure, to provide an annuity to her during her life of $1,500, for her own separate use and benefit.
At her death, the said real estate was to be reconveyed to Thomas Law and his heirs, clear of encumbrances by the annuity administrators.
Eliza P. C. Law acquired a residence in Alexandria, Virginia, and continued to be active in Washington, DC society.

Their daughter Elizabeth Law (19 January 1797 – 9 August 1822) had three children with her husband Lloyd N. Rogers: Edmund Law (1816-1865), Eliza, and Eleanor Rogers.
Elizabeth Law Rogers was survived by her husband and children, and both her parents, who at her death had been divorced for more than a decade.
In 1796 Law married Elizabeth Parke Custis (d. 1832), the eldest granddaughter of Martha Washington and step-granddaughter of George Washington. Soon after their marriage, Law and Elizabeth moved temporarily into a new house built on 6th Street SW as part of development in that area. The Federal-style house became known as "Honeymoon House"; the Laws lived there from about March through August 1796. They next moved to the house they had built for them.
Due to differences, Law and his wife separated in 1804, and they were divorced on 18 January 1811. The couple had one daughter who survived infancy, Elizabeth Law (19 January 1797 – 9 August 1822). She married Nicholas Lloyd Rogers (known as Lloyd N. Rogers) of Druid Hill, Baltimore on 5 April 1817.
When Law and his wife, known as Eliza P. C. Law, separated, he made a deed of certain real estate as jointure, to provide an annuity to her during her life of $1,500, for her own separate use and benefit. At her death, the said real estate was to be reconveyed to Thomas Law and his heirs, clear of encumbrances by the annuity administrators. Eliza P. C. Law acquired a residence in Alexandria, Virginia, and continued to be active in Washington, DC society.
Their daughter Elizabeth Law (19 January 1797 – 9 August 1822) had three children with her husband Lloyd N. Rogers: Edmund Law (1816-1865), Eliza, and Eleanor Rogers. Elizabeth Law Rogers was survived by her husband and children, and both her parents, who at her death had been divorced for more than a decade.
Thomas Law (1756–1834)