Upon the outbreak of the American Revolutionary War Fitzpatrick enlisted in the Continental army. During that service, he was flogged as a punishment for some infraction, and he subsequently deserted. He was arrested and imprisoned in Philadelphia, then released upon condition to re-enter the army, whereupon he deserted a second time. After General Howe captured Philadelphia in 1777, Fitzpatrick joined the British army and, on a number of occasions, engaged in raids into the area now known as Chester County and Delaware County. After the British army evacuated Philadelphia in 1778, Fitzpatrick remained in the area and continued to engage in raids against the Continental Army and its supporters in the area. Fitzpatrick particularly preyed on tax collectors—after robbing them, he would frequently strip them, tie them to trees and flog them. On another occasion, he captured a Continental officer who prided himself on wearing his hair in a neat queue—after robbing him and beating him, Fitzpatrick cut the officer's hair as an additional indignity.