Mackesy was educated at Wellington College in Crowthorne and was commissioned into the Royal Scots Greys in 1944, serving until 1947.
degree at Oriel College, Oxford, where he wrote his thesis on British Strategy in the Mediterranean, 1803–1810.
For many years, he taught the special subject in military history at Oxford with Professor N. H. Gibbs.
He taught at the Naval War College in Newport, Rhode Island; the United States Military Academy in West Point, New York; and Northeastern University in Boston.
He was the Lees Knowles Lecturer at Cambridge University in 1972, and served as a member of Council, Institute of Early American History and Culture in Williamsburg, Virginia, 1970–73.