When the lots for the new town of Alexandria were auctioned in July 1749, John Carlyle purchased the lots 41 and 42, situated between the Potomac River and the town's market square, ideal for his merchant business.

King George II sent Edward Braddock with two regiments of British regulars (2500 troops) to America to fight in the French and Indian Warthat started in 1754, they arrived on 20 February 1755 in Hampton, in the colony of Virginia. In April 1755 they came to Carlyle house, and it became the initial headquarters for Major-General Edward Braddock in the Colony of Virginia. On April 15, 1755, was held the Congress of Alexandria in which Braddock met with five colonial governors, Horatio Sharpe (Maryland's governor ), Robert Dinwiddie (Virginia's governor), James De Lancey (New York's governor), William Shirley (Massachusetts' governor), and Robert Hunter Morris (Pennsylvania's governor).

They convened in the dining room of the house and here Braddock first suggested the idea of levying additional new taxes on the colonists to help with the cost of the war, and also decided to make an expedition to Fort Duquesne during the French and Indian War. George Washington, who was appointed as major in the provincial militia in February 1753 by Robert Dinwiddie (Virginia's Royal Governor), urged Braddock not to undertake the expedition and became a volunteer aide-de-camp to Braddock. Nevertheless, Braddock decided to undertake the expedition, resulting in the death or injury of two-thirdsrds of Braddock's troops and also in Braddock's own death.