On 29 March 1918, he was commissioned as a second lieutenant into the Queen's Own Cameron Highlanders,[6] continuing to serve in France and Belgium with the 1st Battalion until the end of the war.

[9] From March 1941 Barber returned to the General Staff as a GSO1, until taking command, in October, of the 46th (Highland) Infantry Brigade, leading it through the Battle of Normandy in the summer of 1944.

[5] In this campaign, the 15th Division had the distinction to lead the three great river crossings of the Seine, the Rhine and the Elbe[3] and Barber was awarded the bar to his DSO.

[11] On 13 November 1945, while acting as representative for the Commander-in-Chief British Army of the Rhine, Barber and the Soviet major-general Nikolay Grigoryevich Lyashchenko (Russian: Николай Григорьевич Лященко) signed the Barber Lyashchenko Agreement ((in German), also Gadebusch Agreement) in Gadebusch, redeploying some municipalities along the northern border between the Soviet and British zone of Allied-occupied Germany.

Barber was promoted to lieutenant-general on 27 February 1952 and made General Officer Commanding-in-Chief (GOC-in-C) of Scottish Command and Governor of Edinburgh Castle.