[1] While in Edinburgh Johnston took up portrait-painting, and he brought with him to London some portraits of Dr. Morison's family, which he exhibited at the Royal Academy in 1836 and 1837.

In 1838 he exhibited there his first subject picture, The Mother's Prayer, and sent his Scotch Lovers to the Society of British Artists.

In 1839 his picture of The Mother's Grave at the Royal Academy attracted favorable notice, while The Gentle Shepherd (1840) and Sunday Morning (1841) (formerly in the Bicknell collection and engraved by F. Bromley) established his popularity.

[2] In 1841 Johnston exhibited his first historical picture, The Interview of the Regent Murray with Mary Queen of Scots, which was purchased by the Edinburgh Art Union.

In 1845 Johnston exhibited Archbishop Tillotson administering the Sacrament to Lord William Russell in the Tower, which was purchased by Robert Vernon, formed part of The Vernon Gallery, and went to the National Gallery (engraved by T. L. Atkinson and Charles Henry Jeens).