Jethro Sumner was born in 1733 in Nansemond County, Virginia, the son of Jethro Sumner and Margaret Sullivan. From 1755-1761, he served in the Virginia Militia; he was promoted to lieutenant in 1758, commanded Fort Bedford in 1760, and ended the French and Indian War as a captain. In 1764, he moved to Warren, North Carolina, where he married Mary Hurst, daughter of a wealthy planter, in 1765. They had three children. He soon opened a tavern and became involved in local politics, first as a justice of the peace in 1768, and then as the Warren County sheriff from 1772 to 1777.
Sumner was elected major in the Halifax district minutemen in 1775, and the following year became a colonel of the Third Battalion of North Carolina Continentals. He participated in the defense of Charleston in June 1776, and served under Washington at Brandywine, Germantown, and Valley Forge. In 1778, he suffered from an illness and returned home to recruit replacements for the Continental units. Sumner was made brigadier general by the Continental Congress in January 1779, and the following June, led a brigade in the Battle of Stono Ferry. In 1780, he joined the defense of North Carolina against Cornwallis' invasion and resigned in protest when General William Smallwood was given command of the state troops in October. General Nathanael Greene persuaded him to return to the field shortly thereafter, and he performed his most important service commanding a brigade that reinforced Greene at Eutaw Springs in September 1781. He retired in 1783, his health seriously impaired, and died two years later.