McKay accepted an appointment under Dorothea Dix as a hospital nurse, and commenced her hospital life in Frederick, Maryland, in March, 1862, where she was entrusted with the care of a large number of wounded from the First Battle of Winchester. In September, 1862, the city was filled for two or three days with Stonewall Jackson's Corps on their way to South Mountain and Antietam. The rebels took possession of the hospital, and filled it for the time with their sick and wounded men. Resistance was useless, and McKay treated the rebel officers and men courteously, and did what she could for the sick; her civility and kindness were recognized, and she was treated respectfully. After the Battle of Antietam, Frederick City and its hospitals were filled with the wounded, but as soon as the wounded became convalescent, she went to Washington, D.C. and was assigned to duty for a time in the hospitals of the Capital. In January, she went to Falmouth, Virginia and found employment as a nurse in the Third Corps Hospital. Here, she soon effected an evolution in the hospital; from being the worst, it became the best of the corps hospitals at the front. General David B. Birney and his wife, seconded and encouraged all her efforts for its improvement.