John Prideaux was born around 1718 into the large and well-established Prideaux family of Devon and Cornwall. Precise details of his birthplace and childhood remain uncertain, but his background placed him among the provincial gentry from whom the British Army drew many of its officers. He entered military service as a young man and began the slow process of advancement through commissions, regimental duty, and wartime experience.

Prideaux served in the 3rd Regiment of Foot Guards and later became associated with the 55th Regiment of Foot. During the War of the Austrian Succession he gained experience in the European style of campaigning, in which fortifications, artillery, supply magazines, and coordinated movements were often as important as open battle. Like many officers of his generation, he learned his profession through practice rather than through a formal military academy.

By the early 1750s Prideaux had achieved field rank and was recognized as a competent, dependable officer. He married Elizabeth Roper, linking himself to another respectable family, and balanced the demands of domestic life with the uncertainty of military postings. When the Seven Years’ War expanded, the army’s rapid growth created opportunities for officers with long service. Prideaux became colonel of the 55th Foot and was promoted to brigadier general.

His assignment to North America required him to adapt European training to a theater dominated by waterways, forest corridors, and alliances with Indigenous nations. Successful operations around Lake Ontario demanded boats, skilled pilots, engineers, artillery, and diplomacy with the Haudenosaunee. Prideaux was not among the most famous British officers before 1759, but his reputation for methodical preparation led to his selection for an independent expedition against Fort Niagara. By then, his career had prepared him to manage a mixed force and a siege far from the principal British bases.