Known as William, he was the son of William Seymour and Lydin Seymour (nee Brooks) of 56 Glen Road, Andersonstown, Belfast. The family were of Church of Ireland faith.
In 1911 the family are living in house 15, Stranmillis Park, Belfast.
Following his education at RBAI (Inst), Belfast, in March 1913, Seymour joined Northern Bank at Head Office. Transfers to Donegall Square (1913) and Head Office (1914) followed.
Whilst working for the bank, he was also a member of the Queen's University Officer Training Corps from 1st November 1915 to 21st January 1916.
Second-Lieutenant William Matthew Seymour, 9th Battalion Royal Irish Fusiliers, received his commission in the 10th (reserve) battalion Royal Irish Fusiliers.
Aged 20, he was Killed in Action on 16th August 1917 on the opening day of the battle of 'Langemarck' 16th August, 1917.
Private Brady of the battalion reported:
"He was in command of VIII platoon. He was a young man and I knew him well; he was fair and very nice and well liked by everyone. I saw him during the attack; we were well over the ridge to the left of St Julien. I was only 10 yards off him when I saw him killed outright by a piece of shrapnel. I was wounded very shortly afterwards. I have heard since that we took our objective that morning but got driven back again, so perhaps the Germans would get his body."
Seymour was awarded the British Victory Medal and the British War Medal and he is commemorated on the Tyne Cot Memorial, Flanders, Belgium.
The Northern Whig of 10th August 1918 reported:
Ulster Division Officer Missing
Second-Lieutenant W.M. Seymour 9th Battalion Royal Irish Fusiliers, reported missing at Ypres on the 16th August, 1917. He was an old boy of the Royal Academical Institution, and left the Northern Bank (Donegall Square West Branch) to join up. His relatives had no definite news, but two or three vague contradictory messages through the British Red Cross Society and other sources. They regard it as possible that he was picked up wounded by the Germans as reported from one source, and that some returned prisoner might be able to give information. Any news would be most gratefully received by his brother, Mr. J. Seymour, Preparatory School. R.A.I., Inchmarlo, Marlborough Park, Belfast.
The Belfast News Letter of 6th January 1919 reported: