After attending Sandhurst, he was commissioned as a Lieutenant in the British Army and found himself in the trenches at the 1917 Battle of Passchendaele,[clarification needed] near Ypres in Flanders, Belgium, during the Great War.

Later in life, in recounting his experiences in that ghastly battle, he described how the greatest danger that the stretcher-bearers and medics faced was the ubiquitous mud.

To step off the board walks—which were necessitated by the conditions—while carrying the dead and wounded from the front was to risk literally drowning in mud.

At the end of his life, as time past and present merged in his mind, he relived the terror of that experience, confusing those around him with his stretcher-bearing comrades and warning them of the treacherous mud.

[1] Harrison joined the Department of Reconstruction for Eastern Macedonia after the First World War, where he was appointed Assistant Architect and Town Planner; his tasks included planning Nigrita and other settlements in Greece.

[1] His next position (from 1923 onwards) was as Chief Architect to the Department of Public Works in the civil administration of British Palestine, which led to him designing various edifices in places such as Jerusalem and Amman.

While the exterior is a wonderful amalgam of modern trends from the 1930s and traditional Middle Eastern themes, the interior is no less inspiring.

Harrison also befriended George Horsfield, the Chief Inspector of Antiquities in Transjordan, and John Crowfoot, the second Director of the British School of Archaeology in Jerusalem.

After a donation to the University of Oxford from Lord Nuffield, Harrison next was appointed as architect for the newly established Nuffield College, Oxford, but the donor rejected his first plans for the college (which were heavily influenced by medieval Mediterranean buildings and traditional Arab design) and refused to allow his name to be associated with them, saying that they were "un-English".

[3][4] Harrison modified the design so that the college looked like "something on the lines of Cotswold domestic architecture", as Nuffield wanted.