[1] He returned to Fauquier County, Virginia circa 1821 and received a plantation from his father as a wedding present.
[4] After his elder brother Thomas's unexpected death in 1835, first his brother Edward Carrington Marshall would run for and win one of the county's two seats in the Virginia General Assembly, and then after Edward fell from a horse and became disabled, James Keith Marshall ran and represented the county from 1839 until 1841 (winning re-election once, but losing in 1838).
In 1853, voters elected James Keith Marshall to the Virginia Senate, where he represented Fauquier and neighboring Rappahannock Counties.
[5] After Virginia declared its secession, Marshall continued to serve in the Senate, but died in office on December 2, 1862.
James Keith Marshall was buried in the family plot in the cemetery of Leeds Episcopal Church.