In the Virginia of George Washington's day, wealth was primarily derived from the acquisition of land. Washington's father, Augustine, was the owner of Mount Vernon, a large, sprawling plantation that generated a considerable income for the Washington family. When George was eleven years old, his father died, and so the onerous responsibility of managing the family estate passed into the youngster's hands.
Young Washington supplemented his income from Mount Vernon with money earned from work as a surveyor. However, the vast bulk of his wealth still came from land ownership. When George Washington married Martha Dandridge Custis, he became wealthier still. Martha was a rich widow with substantial landholdings of her own that she brought to the marriage.
Washington generated additional income from his Mount Vernon estate through significant improvements and additions. In total, Washington owned over 50,000 acres of land, and the wealth he derived from this vast acreage made him the richest president in American history.
Even so, it is estimated that Washington would've been even richer had he not devoted so much time to his military and political careers. This was because he didn't have as much time as was needed to oversee the day-to-day management of his estates as he did in the days before the Revolutionary War.