He was the son of William James Mercer and Margaret Mercer. In 1901 the family were living in house 5, Bridge Street, Dromore, Co. Down. By 1911, the property is described as house 8, Bridge Street, Dromore, Co. Down.
On 28th September 1912, Mercer signed the Ulster Covenant at Dromore.
Mercer would have joined the Belfast Banking Company around 1915 and worked in Derry and Rathfriland branches.
Mercer served with the 26th Bankers’ Bn. Royal Irish Fusiliers. He is also recorded as serving in the 22 Royal Fusiliers (City of London Regiment). Hiss Service Number was G/52005.
The Belfast Banking Company 'Roll of Honour' booklet records Mercer as being a 'prisoner of war'.
Following his demobilisation after the Great War, Mercer returned to work.
Mercer was a member of the Belfast Banking Company Sports Club in March 1948 paying 1 shillings subscription.
Retirement came in 1960.