His brothers Daniel McCook, Jr., Edwin Stanton McCook, and Robert Latimer McCook were all Union generals, as were his cousins Anson G. McCook and Edward M. McCook.
Hans brødre Daniel McCook, Jr., Edwin S. McCook og Robert L. McCook blev alle generaler i Unionshæren ligesom hans fætre Anson G. McCook og Edward M. McCook.