In 1755, at the age of fourteen, John Smart began to enter pencil and chalk drawings for prizes at the Society of Arts. After coming in second to Richard Cosway in the first competition, he went on to secure first prize in the following three. The 18th Century was a highly prosperous period for the miniature painter, and witnessed the success of Cosway and Engleheart to name just two.

During the 1760s, Smart was a rival to the equally young Richard Cosway but his miniatures are markedly different; although both artists were painting portraits on a similar scale, Cosway opted for a more virtuoso style of painting, which he executed with a lightness of touch. Smart, however, embraced a more delicate, meticulous style and his sitter's costumes tend to be more restrained.

These early years provided Smart with the necessary confidence, contacts and drive, and in 1765 he was elected a Fellow of the Royal Society of Artists (FSA), becoming Director in 1772, Vice-President in 1777 and finally President in 1778, a position he held until the Society's liquidation. By 1773, Smart had been working as a professional artist for six years, practicing from premises in Dean Street, Soho, London. The business appears to have been remarkably successful in a short time, with Smart working hard to secure large numbers of sittings.

His confidence and desire for recognition can be gauged by his initials appearing on even his earliest works, followed by a date. John Smart continued to gain much fame for his miniatures, moving to India in 1785 to secure new, wealthy clients. He came back to London in 1795 and quickly re-established himself as one of the most talented miniaturists in the country, exhibiting many works at the Royal Academy. He died after a short illness at his home in Russell Place, Fitzroy Square, London on May 1, 1811.