Mackellar first saw active service when he took part in the Braddock Expedition (along with the young George Washington) against Fort Duquesne (now Pittsburgh, Pennsylvania) in 1755. He was severely wounded in the Battle of the Monongahela on 9 July, when Braddock and 976 British and provincial soldiers out of the 1459 deployed were killed, but by the following spring he was at Fort Oswego as chief engineer of the frontier forts. During the summer Mackellar was engaged in repairing and strengthening the obsolete fortifications at Oswego, a difficult task as the original defences had been poorly sited and laid out. On 11 August a French force of 3000 men under the command of the Marquis de Montcalm appeared and the Battle of Fort Oswego (1756) ensued. The British surrendered during the afternoon of 14 August and Mackellar, having survived the subsequent massacre perpetrated by Montcalm's Indian allies, was taken to Quebec City where he was kept closely guarded. He was able, however, to make detailed notes on the city's defences and his report, bearing the initials "PM" and of which several copies still exist, is dated September 1756. At some time in September he was transferred to Montreal and after some months as a prisoner-of-war, Mackellar was exchanged. He returned to Britain early in 1757 and on 14 May, when all military engineers were finally given army rank, he was commissioned a captain in the Corps of Engineers. In December 1757, following a meeting with a fellow ex-prisoner-of-war named Moss, it would appear that Mackellar had time to draw upon his notes in order to compile a postscript to his report, which was dated and initialled 23 December 1757, and to arrange the production of a map. It was undoubtedly his recent experiences in New France and the important intelligence contained in his report that prompted his next appointment.