In his early years Isaac was a page to Lord Chesterfield, the Lord Lieutenant of Ireland which allowed him to mix with fashionable Dublin society. When Chesterfield was replaced in the position in 1745 he arranged for Isaac to be given a commission in the army. In October 1745, Bickerstaff joined the 5th Regiment of Foot known as the Northumberland Fusiliers. He served as an Ensign until 1746, when he was promoted to Lieutenant. The regiment, under the command of Alexander Irwin, was on the Irish Establishment and was based in Kinsale in Ireland. In March 1755, the regiment was moved to Bristol in England. Having recently come into some money, Isaac resigned his commission in August and went on half-pay.