Gardnor began life as a drawing-master, teaching drawing, painting, and calligraphy.
In 1763 he exhibited with the Free Society of Artists, sending two drawings with a specimen of penmanship.
In 1778 he was instituted to the vicarage of St Mary's Church, Battersea, which he continued to hold up to his death.
On 16 May 1787 Gardnor started with his nephew Richard on a tour to Paris, Geneva, Lausanne, Basle, Strasburg, and back down the Rhine.
These views were engraved in aquatint by Elizabeth and William Ellis, Robert Dodd, Samuel Alken, J. S. Robinson and by Gardnor himself.