 On January 16, 1866, President Andrew Johnson nominated Allen for the award of the brevet grade of major general to rank from March 13, 1865 and the U.S. Senate confirmed the award on March 12, 1866.On July 17, 1866, President Johnson nominated Allen for the award of the brevet grade of major general, U.S. Army, also to rank from March 13, 1865, and the U.S. Senate confirmed the award on July 23, 1866.He was mustered out of the volunteer service on September 1, 1866.