John McDonald was born in Rochester, New York, on February 22, 1832. At the age of nine, he was orphaned. To support himself, McDonald worked a series of odd jobs on canals, lakes, and rivers. He arrived in St. Louis, Missouri, at the age of 15. McDonald worked his way up in responsible river trade jobs. By the 1850s, he was a passenger agent for a St. Louis steam company. Eventually, he started his own business, running a freight steamer that carried passengers on the Missouri River. At the outbreak of the American Civil War, McDonald was a strong Union man. Appointed a Major rank, McDonald raised and outfitted the Eighth Regiment that served in battles at Fort Henry, Fort Donelson, and Shiloh. President Abraham Lincoln appointed him a brigadier general. McDonald kept the title of "General" for the rest of his life.