During the Battle of Mansfield at De Soto Parish, Louisiana, McMillan and the rest of the 1st Division were able to provide a steady line to rally on for the near-panicking Union retreat.

[1] That July, the XIX Corps was sent east to the Valley of Virginia, ordered to join Maj. Gen. Philip Sheridan's Army of the Shenandoah that was assembling there.

On October 19, McMillan fought with distinction during the Battle of Cedar Creek, where he again arranged his men to rally fleeing Union soldiers, providing a place to regroup for the coming rout of Lt. Gen. Jubal A.

[1] After the campaign in the Valley, McMillan was ordered to lead a "provisional" or temporary division in March 1865, with his headquarters in Grafton, West Virginia, until the end of the war that April.

James McMillan died there on March 9, 1903[3] in Washington, and was buried with full military honors in Arlington National Cemetery in Virginia.