Documents dated July 25, 1782, state that Johnson was the "son of a white man and a black slave woman owned by a William Wheeler, Sr." His father, George Johnson (also spelled Johnston in some documents), purchased Joshua, age 19, from William Wheeler, a small Baltimore-based farmer, confirmed by a bill of sale dating from October 6, 1764.
Wheeler sold Johnson the young man for £25, half the average price of an enslaved male field hand at the time.
He moved frequently, residing often where other artists, specifically chair-makers, lived, which suggests that he may have provided extra income for himself by painting chairs.
No records mention educational or creative training, and it still has not been proven that he had any relationship with artists such as the Peale family, Ralph Earl, or Ralph Earl Jr.[2] Catholic Church records show that in 1785, he married his first wife, Sarah, with whom he had four children – two sons and two daughters, the latter of whom both died young.
In his advertisement in the Baltimore Intelligencer of December 19, 1798, Johnson called his portraiture the work of "a self-taught genius, deriving from nature and industry his knowledge of the Art.
Both were noted for painting multi-figure family group portraits, which were rare during this period of American art.