 Fitzgerald joined the British Army and became a lieutenant in the 66th Foot in 1788, transferring as a captain in 1779 to the newly raised 85th Foot, which was posted to garrison duty in Jamaica for the duration the American Revolutionary War.He was there promoted to major in 1781 and lieutenant-colonel in 1783, taking over command of the regiment from General Charles Stanhope, 3rd Earl of Harrington.After the 85th was disbanded in 1783 he became a captain and lieutenant-colonel in the 2nd Foot Guards in 1789 and retired from active service in 1792.