John was the 7th child and youngest son of Neill McKay and Flora McNeill of Cumberland (later Harnett) County, North Carolina. His paternal grandparents were Archibald McKay and Ann Gilchrist, Scottish immigrants who came to the country in the 1750s. His maternal grandparents were Malcolm McNeill and Jennet McAlester, daughter of Alexander McAllister who was Colonel of the Cumberland County, North Carolina militia (Patriot) during the Revolutionary War. John graduated from Princeton in 1837 and subsequently attended medical school at the University of Pennsylvania. In 1842, he began his medical practice in Cumberland County, North Carolina. In 1846 he married Mary Elizabeth James, daughter of Rev. Robert Wilson James of Sumter County, South Carolina. Dr. McKay continued to practice medicine until 1852, when he handed over his practice to his brother-in-law, Dr. Murdock McCloud and entered the agriculture and lumber and naval stores businesses. During the War Between the States he was a member of the Home Guards and was appointed by Gov. Vance to furnish supplies to the Confederate Army. After the War, he moved to Rockingham, North Carolina and, around 1870, he sold off the land he owned in North Carolina and moved to Cheraw in Chesterfield County, South Carolina. By 1892 he owned 50,000 acres of timber land, employed over 100 people in his turpentine distilleries, was President of Cheraw Iron Works, and was Vice-President of the Bank of Cheraw. He belonged to the Masonic fraternity and was an elder for many years in the Presbyterian Church. John was the father of 8 children, 5 of which survived to adulthood - Rev. Wilson James McKay (of Mayesville, SC), Sarah Lavinia McKay (wife of Capt. Robert Motier McIntire, a Merchant of Wilmington, NC), Flora Margaret McKay (of Cheraw, SC), Mary Isabella McKay (wife of Theodore F Malloy, a Merchant and later Mayor of Cheraw, SC), and Robert Murphy McKay (of Cheraw, SC).