[5] During the French and Indian War, in 1755 he was in the Connecticut Regiment, under the command of Sir William Johnston, when he fought in the Battle of Lake George.
He became a major in General Phineas Lyman to capture the Spanish Caribbean Islands, the campaign that concluded the war.
His friend Montfort Browne had been taken prisoner on New Providence by Commodore Esek Hopkins of the Continental Navy on 3 March 1776.
While Montfort Browne was imprisoned in Middletown, Hierlihy raised the Prince of Wales’ American Regiment in Long Island (July 1776).
)[9] In September 1776, while Browne was imprisoned, Hierlihy risked his life to deliver the plan to raise the regiment to the Commander in Chief Howe.
According to Hierlihy's account, the route included crossing Long Island Sound “in the dead of Night, Twenty Six Miles wide in a little open leaky Boat, passing by a Rebel Battery (Fort Black Rock) of 23 Guns.” Howe was so impressed with Hierlihy’s plan that he immediately negotiated with George Washington to have Browne released from prison.
In 1777, he created the Independent Companies (Hierlihy's Corps; 2nd Battalion, Prince of Wales American Volunteers).
On 7 April 1778, Major Hierlihy arrived in Halifax, Nova Scotia in command of his Independent Companies.
He and his unit were on board HMS Hope and were sent to protect and work in the coalmines at Sydney Cape Breton.