Now a Lieutenant, MacDowell and 38th Battalion began their war with a period of Garrison duty in Bermuda from August 1915 until May of the following year where they were succeeded by a French-Canadian Unit. The 38th proceeded directly to England and crossed the Channel to the front in early August joining 12th Brigade 4th Canadian Division in Belgium.
It wasn’t long before they were in the trenches and under fire but their initiation in the attack had to wait a short time. At the end of September, they moved south to join the Canadian Corps in the Somme operations and on the 18th of November went over the top for the first time. Newly-promoted Capt MacDowell was in the thick of battle and led his Company’s soldiers with dash and daring. Their advance was impeded by German Machine Gun fire but MacDowell was able to move close enough to silence not one or two but three MG nests by single handed use of grenades. He closed within throwing distance of the excellently sighted and murderously efficient positions and destroyed them aiding greatly in the capture of all their objectives. The course of the day’s battle was costly with the 38th having some 500 casualties killed or wounded, Thain MacDowell among the latter. He had been wounded slightly in the hand that day but also had been “blown up” by a shell that evening receiving a concussion. These wounds were to cause him great discomfort and he was evacuated to England to recover. For his Bravery and courage in that attack, Captain MacDowell was awarded the Distinguished Service order or DSO, an award just beneath the Victoria Cross in importance. In January 1917, he re-joined the 38th Battalion and was an Acting Major in the trenches in front of Vimy Ridge as the unit prepared for the Battle.
On the 9th of April 1917, the 4th Division was on the far left of the Canadian Corps and had the least distance to cover to their objectives. A lot of his was uphill in tightly constrained with trenches, wire, Machine Guns and concrete pillboxes. MacDowell had studied the maps and photos of the front line carefully and was meticulous in his own planning. From his previous experiences, he knew it was important to know where one was on the battlefield. His unit’s objective was just over the crest of the ridge itself. As the advance began, MacDowell and two of his Runners or Messenger soldier became separated from the main body of soldiers. MacDowell knew where he was so opted to press on. Spotting a Trench work that he thought would be a good HQ he was dismayed to find it covered by two German Machine Gun positions. MacDowell attacked one with grenades and destroyed it so effectively that the other gunner ran away in fright down an underground dugout. Following, Capt MacDowell and his two soldiers went down some 50 steps of tunnel before being confronted by 2 German officers and 75 of their soldiers. Thinking quickly and using the fact that the Germans could not see up the tunnel, Captain MacDowell made that he had a much larger force behind him and the Germans promptly surrendered. The 38th gained all their objective that day, more than 400 casualties was the bill.
Thain MacDowell was again slightly wounded in this action and when his unit came out of the line two weeks later, he was evacuated back to England. In June of 1917 it was announced that he had been awarded the Victoria Cross for his heroic actions on 9 April. He did return to the front briefly but was evacuated again and returned home to Canada to recover from the wars many effects. He did not return to the front again. Thain MacDowell had a successful business career after the war and remained in uniform as an Honorary Colonel. He died in the Bahamas in 1960.