McKinly, born in Ireland, moved to Wilmington, Delaware and established a successful medical practice. He held several important posts and positions in Colonial Delaware, serving variously as sheriff, chief Burgess of Wilmington, Assembly member, and as a member of the Committee of Correspondence. He played an active role in the militia as well. In February 1777, McKinly was chosen Commander and Chief and President of Delaware.
After the British were victorious at the Battle of Brandywine, General Howe sent several brigades of troops to occupy nearby Wilmington. In the early morning hours of September 13, 1777, President McKinly was taken prisoner by these troops. Initially the British imprisoned him in Philadelphia, but then took their valuable prize to New York City after the city was abandoned in June 1778.
McKinly was paroled in August 1778, and shortly thereafter exchanged for William Franklin, Loyalist Governor of New Jersey, and Benjamin Franklin's son. After his release, McKinly shunned political life, instead choosing to resume his medical practice. Though he was subsequently elected to the Continental Congress, he opted not to serve.