Henry Middleton's public career rested on plantation wealth. Inherited estates and later acquisitions made him one of South Carolina's largest landowners and enslavers, with holdings across several counties. He served as a justice of the peace and entered the Commons House of Assembly in 1742, holding the speakership during several sessions. Appointed to the royal Council in 1755, he remained there until resigning in 1770 as opposition to British policy strengthened.

In 1774 South Carolina sent Middleton to the First Continental Congress. He briefly succeeded Peyton Randolph as its president in October, presiding over the final days before adjournment. He returned for the Second Continental Congress in 1775, then concentrated on South Carolina affairs. He served in its provincial congresses and Council of Safety and helped prepare a new form of government in 1776. His son Arthur, rather than Henry, signed the Declaration of Independence.

Middleton sat on the state's Legislative Council from 1776 to 1778 and in its Senate from 1778 to 1780. Alongside these offices he maintained The Oaks and Middleton Place within a plantation economy dependent on enslaved labor. After British forces captured Charleston in 1780, he accepted royal protection and withdrew from revolutionary politics. His decision differed from Arthur's continued commitment and imprisonment, showing family divisions under occupation. He died in Charleston in June 1784 and was buried at St. James Goose Creek.