Suffering from dwarfism, and with severe curvature of the spine, Mary Thompson overcame these difficulties to become widely known for her portrayals of the Welsh slate and granite quarries. Born in Braunton, Devon, she attended a Quaker boarding school in Darlington and then studied at St Albans School of Art (1913-14) and in St Ives under Alfred HARTLEY. She also trained under the noted Belgian Artist Emile FABRY who was in England during WWI, and he recommended that she attend the Academy in Brussels. Between 1919-1922 she studied drawing and sculpture, but finding it physically impossible to pursue her love of sculpture, she chose drawing as her medium (using carbon, rather than lead pencils). She returned to live in North Wales, moving eventually to Bethesda in 1937.

She joined STISA, perhaps on a re-visit to St Ives, and her drawings of Welsh mountain scenes were praised for their perfect draughtmanship. In 1939 the Army took over the mountain slopes for firing practice and she began drawing in the Penrhyn slate quarry, depictions of quarrying becoming her principal focus. She completed a series of drawings covering the whole of the slate industry. In 1954 she moved to join her family in Tunbridge Wells, Kent. Her decreasing mobility caused her to abandon drawing after 1969. In 1981 a booklet was produced to accompany a Welsh Arts Council touring show, An Artist in the Quarries.