John Bernardi's "restless" temperament, along with what he claimed was "harsh" treatment by his father, led him to run away from home at the age of 13. He later wrote that on going through the gate he "kneeled down protesting and praying that he might never return, whilst his father lived". After some time staying with Sir Clement Fisher, he eventually came to enlist as a common soldier in William of Orange's service, later exchanging into English service and eventually rising to the rank of Captain due to his bravery and "uncommon talents". He was wounded in 1674 at the Siege of Grave, received another wound preventing a duel, and at the Siege of Maastricht lost the sight in an eye and was shot through the arm, being rescued after being left for dead on the battlefield.