At the time of Grant's enlistment, the main contingent of the NZEF, which had formed the New Zealand and Australian Division, was engaged in the Gallipoli Campaign.
After initial training, he embarked for the Middle East in October 1915 with the 7th Reinforcements.
He joined the 1st Battalion, Wellington Regiment of the New Zealand Division, then being formed in Egypt in the aftermath of the evacuation from the Gallipoli Peninsula.
It embarked for the Western Front in March 1916.
Grant served on active duty throughout 1916 and into 1917, during which time his battalion fought in a number of battles, including the Battle of Flers–Courcelette, where about a quarter of its strength became casualties.
It also fought in the Battle of Messines, where it was involved in the encirclement of Messines village and captured 200 German soldiers.
 During its time in the battle, it suffered over 400 killed and wounded.
In June 1917, he was promoted to corporal.
A few months later, his battalion was one of the leading units in the Battle of Broodseinde.
It missed the First Battle of Passchendaele that followed a few days afterward and avoided the heavy casualties that befell the units involved.
In early 1918 he was made a sergeant.

From late August to early September 1918, the New Zealand Division was engaged in the Second Battle of Bapaume, which had as its objective the town of Bapaume.
On 1 September, near Bancourt, the lead elements of the 1st Battalion came under heavy fire from a series of German machine-gun posts, which threatened their advance.
Despite this, Grant's platoon pressed on.
As they neared one of the posts, Grant, followed by another soldier, Lance Corporal Cecil Hill, broke ahead and entered the post, "demoralising" the crew, according to the London Gazette, and allowing his platoon to capture the Germans.
Grant attacked another nearby machine-gun post in similar fashion and soon his platoon, and the rest of his company, were able to put the remaining posts out of action.
His battalion was relieved that evening.
While Hill was awarded the Distinguished Conduct Medal for his part in the operation of 1 September, Grant was awarded the Victoria Cross (VC).
The VC, instituted in 1856, was the highest gallantry award that could be bestowed on a soldier of the British Empire.
The citation read:
For most conspicuous bravery and devotion to duty near Bancourt on the 1st September, 1918, when Serjeant in command of a platoon forming part of the leading waves of the battalion attacking the high ground to the east of Bancourt.
On reaching the crest, it was found that a line of five enemy machine-gun posts offered a serious obstacle to further advance.
Under point blank fire, however, the company advanced against these posts.
When about twenty yards from the posts Sjt.
Grant, closely followed by a comrade, rushed forward ahead of his platoon, and with great dash and bravery entered the centre post, demoralising the garrison and enabling the men of his platoon to mop up the position.
In the same manner he then rushed the post on the left and the remaining posts were quickly occupied and cleared by his company.
Throughout the whole operation on this and the two previous days Sjt.
Grant displayed coolness, determination, and valour of the highest order, and set a splendid example to all.

— London Gazette, No.
31034, 26 November 1918
Shortly afterwards, Grant was promoted to second lieutenant and travelled to Cambridge in England for officer training in October 1918.
He was wounded in early November, within days of his return to the front.
Together with three other New Zealanders who had been awarded the VC, he received his medal from King George V in a ceremony at Buckingham Palace on 27 February 1919.
His service with the NZEF ended with his repatriation to New Zealand later that year.
His home town of Hāwera gave him a formal welcome on his return on 29 October and he was presented with an inscribed gold watch.
At the time of Grant's enlistment, the main contingent of the NZEF, which had formed the New Zealand and Australian Division, was engaged in the Gallipoli Campaign. After initial training, he embarked for the Middle East in October 1915 with the 7th Reinforcements. He joined the 1st Battalion, Wellington Regiment of the New Zealand Division, then being formed in Egypt in the aftermath of the evacuation from the Gallipoli Peninsula. It embarked for the Western Front in March 1916. Grant served on active duty throughout 1916 and into 1917, during which time his battalion fought in a number of battles, including the Battle of Flers–Courcelette, where about a quarter of its strength became casualties. It also fought in the Battle of Messines, where it was involved in the encirclement of Messines village and captured 200 German soldiers.  During its time in the battle, it suffered over 400 killed and wounded. In June 1917, he was promoted to corporal. A few months later, his battalion was one of the leading units in the Battle of Broodseinde. It missed the First Battle of Passchendaele that followed a few days afterward and avoided the heavy casualties that befell the units involved. In early 1918 he was made a sergeant.
From late August to early September 1918, the New Zealand Division was engaged in the Second Battle of Bapaume, which had as its objective the town of Bapaume. On 1 September, near Bancourt, the lead elements of the 1st Battalion came under heavy fire from a series of German machine-gun posts, which threatened their advance. Despite this, Grant's platoon pressed on. As they neared one of the posts, Grant, followed by another soldier, Lance Corporal Cecil Hill, broke ahead and entered the post, "demoralising" the crew, according to the London Gazette, and allowing his platoon to capture the Germans. Grant attacked another nearby machine-gun post in similar fashion and soon his platoon, and the rest of his company, were able to put the remaining posts out of action. His battalion was relieved that evening. While Hill was awarded the Distinguished Conduct Medal for his part in the operation of 1 September, Grant was awarded the Victoria Cross (VC). The VC, instituted in 1856, was the highest gallantry award that could be bestowed on a soldier of the British Empire. The citation read:
For most conspicuous bravery and devotion to duty near Bancourt on the 1st September, 1918, when Serjeant in command of a platoon forming part of the leading waves of the battalion attacking the high ground to the east of Bancourt. On reaching the crest, it was found that a line of five enemy machine-gun posts offered a serious obstacle to further advance. Under point blank fire, however, the company advanced against these posts. When about twenty yards from the posts Sjt. Grant, closely followed by a comrade, rushed forward ahead of his platoon, and with great dash and bravery entered the centre post, demoralising the garrison and enabling the men of his platoon to mop up the position. In the same manner he then rushed the post on the left and the remaining posts were quickly occupied and cleared by his company. Throughout the whole operation on this and the two previous days Sjt. Grant displayed coolness, determination, and valour of the highest order, and set a splendid example to all.
— London Gazette, No. 31034, 26 November 1918
Shortly afterwards, Grant was promoted to second lieutenant and travelled to Cambridge in England for officer training in October 1918. He was wounded in early November, within days of his return to the front. Together with three other New Zealanders who had been awarded the VC, he received his medal from King George V in a ceremony at Buckingham Palace on 27 February 1919. His service with the NZEF ended with his repatriation to New Zealand later that year. His home town of Hāwera gave him a formal welcome on his return on 29 October and he was presented with an inscribed gold watch.
John Gildroy Grant