He was the brother of Alexander and Archibald Robertson, who were also painters.

[1] Robertson's self-portrait hangs in the National Portrait Gallery, London.

[2] Robertson created a new style of miniature portrait that became dominant by the middle of the nineteenth century; at least four examples are held in the Victoria and Albert Museum.

[3] He broke with previous styles, particularly the work of Richard Cosway, and was critical of these earlier painters, describing their works as 'pretty things but not pictures'.

[3] Robertson's style included larger and more detail paintings, usually rectangular, and with a use of paint trying to emulate large oils on canvas, adding more gum to the paint to give it a greater lustre and depth of colour.