John McLean was born on June 23, 1854, in Cincinnati, Ohio, to Nathaniel C. McLean and Caroline Thew. His father was a lawyer who owned $150,000 of real estate and $27,000 of personal property by 1860. His mother died sometime in the 1850s. He grew up and attended school in Cincinnati. He travelled abroad in 1878, and his passport application described him as 5 feet, 7 inches tall, with brown hair and grey eyes.
He married Jane Elizabeth Riley around 1882, and their daughter Nathalie was born around 1885. They lived in St. Paul, Minnesota, and McLean worked as a doctor. By 1930, he was living in his daughter’s household in Chicago, Illinois. He died there on April 2, 1932.