The newly built ship was transferred to the War Department on 29 October 1920 for operation with the Army Transport Service (ATS) homeported at the Brooklyn Army Base in New York. Until 1923 "St. Mihiel" generally ran between the New York Port of Embarkation and Antwerp, Belgium. After transporting the last United States troop contingent from Germany from Antwerp in January 1923 she was assigned the New York—San Juan, Puerto Rico—Panama until 1926. During this period of Atlantic service "St. Mihiel" was awarded the Army's Efficiency (“E”) pennant for Atlantic transports in 1922, transported the 213th Coast Artillery from New York to Fort Monroe for summer training and engaged in hurricane relief operations a San Juan during September 1928.