He was named for his paternal grandfather, James Garnett (1692–1765), who served in the Virginia House of Burgesses as well as operated plantations in Essex County using enslaved labor.
His grandson Muscoe Russell Hunter Garnett (1821–1864) would become a prominent lawyer, Virginia politician and Confederate congressman long after this man's death.
McGuire, Garnet inherited plantations in Essex County (including Elmwood) and continued the family tradition of farming using enslaved labor, although as discussed below, he would become a leading advocate of scientific agriculture and education.
[9] In 1804, voters in Virginia's 11th congressional district elected him as a Democratic-Republican to the United States House of Representatives, and re-elected him in 1806, so he served from 1805 to 1809.
Despite his dislike for political campaigning, Garnett became a leading advocate for applying scientific techniques to farming, and strove to educate fellow farmers.
[10] In 1820 Garnett led his fellow Fredericksburg area farmers in a protest against recent protective tariffs, then attended several national anti-tariff conventions between 1821 and 1831.
John Peyton McGuire (1800-1869, who had remarried to Judith Brockenbrough) established a school in Essex County and promoted his innovation of report cards.