When George Mason of Gunston Hall's eldest son George Mason V came of age in 1774 his father gave him part of the 5,500 acre Gunston Hall tract. On this tract, called Lexington, George Mason V built his mansion house and estate.

It is not known exactly when the mansion house and gardens at Lexington were constructed. Research points to the probability that the mansion house at Lexington was built some time after George Mason V returned from Europe in 1783. Family letters reveal that the land was farmed from (and probably before) the time George Mason V was given the property in 1774. Tax records indicate that there were outbuildings (perhaps some of them slave quarters) on the property. However, with George Mason V away in Europe and the economic hardships caused by the war, it is unlikely that a dwelling was erected until 1783 or after. Letters do suggest that George Mason and his bride lived at Gunston Hall until sometime between late 1787 and early 1788 when they moved into Lexington.

The inventory taken after George Mason V’s death in 1796 indicates that Lexington, which now consisted of what was previously the Gunston Hall tract of land, contained household appointments second only to Mt. Vernon in Fairfax County. That document records that George Mason V had about 115 slaves at his home house and on the four plantation quarters (approximately 40 at Lexington; 21 at Dogues Neck; 14 at Pohick; 11 at Occoquan; and 19 at Hallowing Point). Among the 40 slaves at Lexington were carpenters, blacksmiths, waiters, maids, a male cook, and a nurse. The inventory also notes large quantities of livestock including sheep, pigs, cattle, oxen, and horses. The extensive entries for horses include "4 carriage horses," "Two Riding horses," and "Sportsman," a stud horse. Only Virginians of great wealth kept carriages.

George Mason V, who suffered from poor health most of his life, died in 1796 just four years after his father. The combined property of Lexington and Gunston Hall were bequeathed in his will to his two eldest sons, George and William Eilbeck, with George given first choice of the property when he came of age. He chose Gunston Hall and so Lexington became the property of William Eilbeck Mason.