Dr. Cornyn served as surgeon of the First Missouri Union Regiment under Colonel Francis Preston Blair. After the battle of Shiloh, he organized the Tenth Missouri Union Cavalry in an effort to end the war as soon as possible. About the same time, however, he was accused of misconduct by a fellow officer. Although he won his case before a military court in Tennesse, his adversary shot and killed him in the courtroom. His military funeral at St. Francis Xavier in St. Louis drew the largest congregation in St. Louis history up to that time.