McCook was born in New Lisbon, Ohio, to Dr. John McCook. He learned the printing trade as a youth, then taught school for several years. attended Jefferson College. He was a member of the Franklin Literary Society at Jefferson College. After graduation in 1859, he studied theology privately and in the Western Theological Seminary in Allegheny, Pennsylvania. With the outbreak of the Civil War, he enlisted as a chaplain in the 41st Illinois Infantry as a lieutenant, and helped tend the wounded. As a minister in Clinton, Illinois, St. Louis, and Steubenville, Ohio, McCook became known for his compassion and intellect, and for his leadership in the movement to create Sunday Schools. In 1869, he became pastor of the Seventh Presbyterian church of Philadelphia, where he lived for the rest of his