In 1848 Sevier and Nathan Clifford, the Attorney General of the United States, were appointed ambassadors to Mexico by President James K. Polk to negotiate the Treaty of Guadalupe Hidalgo at the end of the Mexican-American War.
Sevier was part of the powerful "Family" of Democratic politicians in Arkansas, who included his first cousins: Representative Henry Wharton Conway, Governor James Sevier Conway, and Governor Elias Nelson Conway; brother-in-law Senator Robert Ward Johnson, and son-in-law Governor Thomas James Churchill.