He joined Northern Bank on 10th June 1909 at Head Office. Transfers followed to Mohill (1909), Ballycastle (1912) and Shercock (1914). By 1911, Fitzsimons is boarding in house 58, Main Street, Mohill with a colleague, William Harris Boyd (27). Fitzsimons is described as a Bank Clerk.
On 6th October 1914 (aged 23), whilst based in Shercock branch, he enlisted into the Royal Army Medical Corps as a Private with Service Number 42865. He served with the British Expeditionary Force in France and saw further action during the Suvla Bay landings at Gallilopi and in Salonika.
Following demobilisation, he rejoined the Northern Bank on 23rd April 1919 at Shercock. Transfers followed to Ballycastle (1919) and then in charge of Shercock (1921). Further transfers followed with Dowra (1921), Shercock (1924), Skerries (1925), Head Office (1925), Oldcastle (1934) and Head Office (1935).