Mercer started a school and a chapel in Loudoun County that welcomed black people and continued for a short while after her death.

Although her father and both grandfathers had operated their Virginia and Maryland plantations using enslaved labor, Margaret found slavery immoral.

She corresponded with her Essex County, Virginia paternal relatives, including about the important people who visited Cedar Park, the siege of Baltimore during the War of 1812, and her brother's surviving his military service.

[2] The schooner's Captain Abels remained in Liberia for 13 days and in 1832 wrote a positive letter about his experience, which colonizers published.

Mercer organize the Loudoun chapter of the American Colonization Society, died and his heirs placed Belmont, his 1,000-plus-acre plantation for sale.

[2] Richard S. Mercer may have used some of the sale proceeds in 1858 to erect Parkhurst manor in Harwood, in Anne Arundel County, nearly across the state from George Kephart's decades-long farm near Buckeystown (which is about 28 miles away from Belmont, with a nearly direct crossing of the Potomac River along historic Route 15).

[12] The 1860 Federal census for Loudoun County, Virginia classified George as "Merchant" and his son William as "Tradeing",[13] but the Kephart family (now without George Junior who disappeared from the census but would be buried nearby in 1888, and to which 6 year old Julius had been added, only owned one slave, an 18 year old mulatto woman.

[14] Kephart either provided for his children who chose not to continue the business or encountered financial difficulties by 1858, when the firm's name changed to Price, Birch and Co. (which remained the name when Union authorities converted the Alexandria office in 1862 into a jail for Confederate and other prisoners) and in 1860 the Loudoun Circuit Court ordered various properties including Coton liquidated.

[15][16] In 1864, Virginia abolitionist Moncure Conway published Testimonies Concerning Slavery and specifically criticized Kephart, who died at Belmont in 1869, about two years after his wife.

In 2018, community leaders and politicians from Loudoun County, and Liberia visited St. David's Episcopal Church and School in Ashburn to dedicate a Virginia historical marker in Mercer's honor.