In 1755, Washington served voluntarily as an aide to General Edward Braddock, who led a British expedition to expel the French from Fort Duquesne and the Ohio Country. On Washington's recommendation, Braddock split the army into one main column and a lightly equipped "flying column". Suffering from a severe case of dysentery, Washington was left behind, and when he rejoined Braddock at Monongahela the French and their Indian allies ambushed the divided army. Two-thirds of the British force became casualties, including the mortally wounded Braddock. Under the command of Lieutenant Colonel Thomas Gage, Washington, still very ill, rallied the survivors and formed a rear guard, allowing the remnants of the force to disengage and retreat. During the engagement, he had two horses shot from under him, and his hat and coat were bullet-pierced. His conduct under fire redeemed his reputation among critics of his command in the Battle of Fort Necessity, but he was not included by the succeeding commander (Colonel Thomas Dunbar) in planning subsequent operations.

The Virginia Regiment was reconstituted in August 1755, and Dinwiddie appointed Washington its commander, again with the rank of colonel. Washington clashed over seniority almost immediately, this time with John Dagworthy, another captain of superior royal rank, who commanded a detachment of Marylanders at the regiment's headquarters in Fort Cumberland. Washington, impatient for an offensive against Fort Duquesne, was convinced Braddock would have granted him a royal commission and pressed his case in February 1756 with Braddock's successor as Commander-in-Chief, William Shirley, and again in January 1757 with Shirley's successor, Lord Loudoun. Shirley ruled in Washington's favor only in the matter of Dagworthy; Loudoun humiliated Washington, refused him a royal commission and agreed only to relieve him of the responsibility of manning Fort Cumberland.

In 1758, the Virginia Regiment was assigned to the British Forbes Expedition to capture Fort Duquesne. Washington disagreed with General John Forbes' tactics and chosen route. Forbes nevertheless made Washington a brevet brigadier general and gave him command of one of the three brigades that would assault the fort. The French abandoned the fort and the valley before the assault was launched; Washington saw only a friendly fire incident which left 14 dead and 26 injured. The war lasted another four years, and Washington resigned his commission and returned to Mount Vernon.