Prescott's military career is much more well-documented than his early life. He enlisted in the British Army in 1745 and became an ensign in the 15th Regiment of Foot. On 22 January 1755, he reached the rank of captain in the 15th Foot. With the outbreak of the Seven Years' War, Prescott saw action in the 1757 British raid on Rochefort, France, and in 1758, the 15th Foot sailed for North America to participate in the French and Indian War, an ongoing conflict which had become the North American theatre of the Seven Years' War. Prescott was present at the 1758 Siege of Louisbourg. He became an aide-de-camp to General Jeffery Amherst in 1759, participating in the capture of Montreal. Afterward he served under General James Wolfe. He also delivered despatches to England announcing the fall of Fort Lévis in 1760, which was near where the town of Prescott, Ontario would later be founded and named in his honour.