Dunning was appointed solicitor general in 1768, probably at the instance of Lord Chancellor Camden, who as chief justice of common pleas had upheld Dunning's argument against general warrants (in Wilkes's case). In the same year, Dunning's election to Parliament was secured by Lord Shelburne.

When Camden was dismissed (Jan. 16, 1770), Dunning resigned his office in protest and thenceforth was allied with the opposition. In 1778 he supported a bill to relieve Roman Catholics. In 1780 his motion against the king, which stated that the “influence of the crown has increased, is increasing, and ought to be diminished,” was passed by a vote of 233 to 215. Shortly thereafter (April 24, 1780), he failed with a motion that the House of Commons should not be dissolved or prorogued until the constitutional balance favouring Parliament was restored. In 1782, when the 2nd Marquess of Rockingham became prime minister, Dunning was raised to the peerage.