The younger Hopkins worked on his father's farm, and his father relocated to Bennington, Vermont in the 1770s. The younger Hopkins gave up farming after an injury, studied medicine with Dr. Jonas Fay, and became a physician. He served as a surgeon in the militia during the American Revolution, and took part in the Battle of Bennington and the Battles of Saratoga. Hopkins later went to sea as surgeon's mate aboard a privateer, the "New Broom". In October, 1778, the "New Broom" was captured by the British, and Hopkins was briefly held as a prisoner in Newport, Rhode Island; he was quickly exchanged, after which he returned to Bennington.