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 22nd Royal Fusiliers (City of London Regiment) and also the 23rd Royal Fusiliers. His Service Number was G/52005.
The Belfast Banking Company 'Roll of Honour' booklet records Mercer as being a 'prisoner of war'.
Mercer was awarded the British War Medal and the Victory Medal.
According to the UK Pension Records, Mercer was suffering from Myalgia (see below). On 15th May 1919, Mercer was demobilised to the Belfast Banking Company, Dungannon. Between the periods 1922 to 1927 the Pension Records card was updated many times.
Mercer was a member of the Belfast Banking Company Sports Club in March 1948 paying 1 shillings subscription.
Retirement came in 1960.
An Ancestry.Com family tree states that Mercer died on 24th July 1982. This has been confirmed in the Northern Bank staff magazine 'The Link' of Autumn 1982.