In April 1862, Sutherland was promoted to major and assigned to Fort Warren, Massachusetts, where Union soldiers guarded Confederate prisoners of war. Later that year Sutherland traveled west, and was assigned as chief medical purveyor for Union Army units in and around Corinth, Mississippi. He subsequently established medical supply depots in Columbus, Kentucky and Memphis, Tennessee, followed by setting up army field hospitals in Mississippi and Tennessee, as well as a floating hospital on the Mississippi River which was used by the Army of the Tennessee during the Vicksburg Campaign. During the Siege of Vicksburg, Sutherland served on the staff of Army of the Tennessee commander Ulysses S. Grant as assistant medical director and inspector of camps and transports. He took part in the Battle of Jackson and the Battle of Champion Hill, and was commended by Grant for effective medical support and efforts to improve camp hygiene, which caused the number of illnesses and deaths from malaria, smallpox and other diseases to be lower than had been the case in previous campaigns.