Johnson earned his freedom in 1782 after completing an apprenticeship with a Baltimore blacksmith named William Forepaw or Forepaugh. This was not unprecedented; in the 18th century, a small number of slaves and free African Americans worked as apprentices to white artisans. Before 1760, however, most trained craftsmen were either carpenters or cask repairmen. By the 1770s, they were engaged in a number of trades including shoemaking, blacksmithing, and tailoring.
During his time as a blacksmith, Johnson likely learned how to construct canvases and source painting materials.According to property records, Johnson moved often. He primarily lived in a section of Baltimore that was popular with furniture decorators, which suggests that he may have supplemented his income by painting furniture.