Source: Joined battalion per War Diary entry June 20, 1916.
Attended St. Andrew’s College, Aurora, Ontario and may have worked for Imperial Oil after the war.
Duty Nobly Done describes an action in which Lieutenant Sykes was involved:
In the interlude between the two stages of the barrage, the Germans attempted a spoiling attack on the positions of the 18th and 20th Battalions. A few of them managed to reach the jumping off trench before Lieutenant Hugh Sykes and some of the 18th Battalion men with help from Lieutenant Gidley and some bombers of the 19th, in support of the two forward battalions, drove them off. In one case, Private David Egan picked up a Lewis gun and moved out of the trench [during the September 15, 1916 attack at the Somme] to a position from which he could enfilade the attacking Germans, causing many casualties among them and forcing the survivors to withdraw.
Antal, S., & Shackleton, K. (2006). Courcelette: Taking a Turn On the Somme. In Duty nobly done: The official history of the Essex and Kent Scottish Regiment (1st ed., p. 211). Windsor, ON: Walkerville Pub.
The 21st Battalion War Diary further relates this action:
During the night the movement of the 18th , 20th & 21t Battalions to the jumping off trenches, was carried out satisfactorily, perfect silence being maintained. The 19th Battalion had been relieved in the Front Line trench by some Platoons of the other Battalions detailed for the assault. At about 3. am a determined attack by Strong enemy Bombing parties was mad upon both flanks of the right sector of our Trench. This was the section that had been taken from the enemy by the 1st Canadian Infantry Brigade a few days previously. From copies of his orders for this operation, subsequently captured, it appears evident that the enemy had carefully planned an attack at this point. On the right he was met by vigorous bombing and rifle fire, and did not succeed in getting into our trenches. He left many dead & some wounded who were afterwards brought in. A young Subaltern, Lieut Hugh H. Sykes, 18th Battalion promptly organized his bombing defence and effectually maintained his position on the Right. The enemy were unable to gain access to our trenches on that Flank. On the left flank, at a point where the trenches converged, and the Lines were only a few yards apart, the enemy succeeded in rushing a party into our trenches. It would appear that this was to have been but the preliminary to a more determined attack to follow if the rush proved successful. Prompt action on the part of Lieut Gidley, 19th Battalion and a party of 19th Battalion Bombers, assisted by parties from the 20th Bn., quickly drove out the enemy and enabled us to complete plans for our advance.
Lt. Hugh Harding Sykes, Inf.
For conspicuous gallantry in action. He held a section of trench and repelled an enemy bombing attack with great courage and determination. Later he gallantly led his platoon forward, and captured an enemy trench. He was severely wounded.
The London Gazette
Publication date: 14 November 1916
Supplement: 29824 Page: 11081
From the Annals of UTS Schools, 1914-1916 – “Old Boys Decorated for Valor”:
LIEUT. HUGH H. SYKES
Hugh Harding Sykes was educated at the University Schools. He qualified for a commission and was appointed to a lieutenancy in the 35th Battalion. Early in the spring of 1916 he was transferred to the 18th Battalion and went to the Ypres salient in June. Later the division was moved to the Somme where he was severely wounded on September 15th. 
For gallantry on the field he was awarded the Military Cross, the investiture taking place at Buckingham Palace on November 3. 
Lt. Hugh H. Sykes’ name appears on the bronze memorial plaques located in the lobby of UTS Schools, Toronto, Ont.
His portrait appears in the Annals booklet1.
1M. I. Pirie, Private Collection.