McKinly was elected Sheriff of New Castle County in 1757, served 4 three-year terms as Chief Burgess of the town of Wilmington between 1758 and 1776. He also represented New Castle County in the Assembly of the Delaware or Lower Counties from the 1771/72 session through the 1775/76 session. In the events leading up to the American Revolution, he became a member of the Delaware Committee of Correspondence in October 1773, and was chairman by November 1774. Meanwhile, he served as Brigadier General of the New Castle County militia.