He was the son of Thomas F Jenkins and Mary E Jenkins. In 1901, the family were living in house 94, Oldcastle, Co. Meath. By 1911, Jenkins had moved to house 1, Rahaghy, Moylagh, Co. Meath.
On 6th April 1911, he joined Northern Bank in Head Office. He was of Church of Ireland faith. Transfers followed with Dromore (1911), Bailieborough (1912) and Ball’s Branch (1914).
Whilst he was working in Ball’s Branch on 22nd October 1914, Jenkins, aged 20, enlisted into the Royal Dublin Fusiliers as a Private with Service Number 15826. He was gazetted as 2nd Lieutenant in June 1917 and later promoted to Lieutenant in January 1919 with the Durham Light Infantry. Jenkins saw action in the Gallipoli Campaign and in the subsequent campaign against the Bulgarians in Salonika,
On 14th February 1919, Jenkins transferred to the Military Accountancy Department, India. Demobilisation came in December 1919. He was awarded the 1914/15 Star, the British Victory Medal and the British War Medal.
Following demobilisation, Jenkins rejoined the Northern Bank on 21st January 1920 at Head Office. Transfers followed with Ball’s Branch (1920), Head Office (1921) and Ball’s Branch (1922).