The first responsibility for Ninth Army, upon its arrival on 5 September was to take part in the final reduction of the German forces holding out in the French port of Brest.

After the surrender of the town fifteen days later, Ninth Army was sent east to take its place in the line.

On 2 May 1945, the whole of Ninth Army's front reached the agreed demarcation point with the Russians, and the advance ceased.

On 8 July Ninth Army headquarters had moved to Deauville, France to prepare for re-deployment to the United States.

On 27 July the headquarters boarded the Army transport ship John Ericsson bound for New York City, where it arrived on 6 August, the same date of the atomic bombing of Hiroshima.

The headquarters then was moved to Camp Shanks, New York for administrative processing prior to deployment to China.

The headquarters was moved to Fort Bragg, North Carolina in mid-September for demobilization and was inactivated on 10 October 1945.