[7] While he advertised his businesses as being organ making, engraving, and furniture merchant,[8] he also worked as a japanner, painted coats of arms, and published books.
[17] Johnston is also known to have built organs for Boston's Deblois Concert Hall and for St. John's Church (in Portsmouth, New Hampshire).
[18] After Johnston's death, his estate included various items from his many businesses, among them an incomplete organ, numerous pictures, various paintings, artist supplies, and copper plates for engraving and printing.
[23] Johnston's various engravings include prints of scenes, business cards for tradesmen, legal certificates, currency, and even noted music scores.
[24] One of his apprentices was John Greenwood, whose varied artistic career included an early period assisting Johnston with painting and engraving.
[25] Johnston engraved the first known print of an historical event in the colonies,[26] an overview of the Battle of Lake George.
[31] Johnston's print was reprinted in London by Thomas Jefferys, being published six weeks later in February 1756,[32] along with an explanatory eight-page pamphlet.
[36][37] Three of Johnston's sons by his first wife Rachel — Thomas Jr., William, and Benjamin —all became artists or craftspeople of varying kinds, their skills including japanning, portrait painting, organ-building, and engraving.