In 1774 Thomas was a delegate to the state convention that elected his brother Caesar to be their delegate to the Continental Congress. Caesar went on to sign the Declaration of Independence. Meanwhile, Thomas was named to the state's "Committee of Safety". Thomas in turn was sent as a delegate to the Congress in 1781 and 1782. He was elected to the Congress annually from 1785 to 1787, but attended sessions only in 1786. Through these same years Thomas was also a member in Delaware's state Assembly, and served as its Speaker of the House in 1787.