Martina Thomas was born in north London and studied at St Martins School of Art. Though she never lived in Cornwall, she was a regular summer visitor to the county over many years. She and her husband, Eric James Mellon (an artist and ceramicist) and their children spent many happy holidays exploring the coastline of west Cornwall. During the winter months back home at her studio in Bognor Regis, she would work these sketches into fully composed paintings. While her early paintings were characterised by muted earth tones, visits to the south west, Spain and Italy resulted in a brighter palette and her later works possess a shimmering vibrancy reminiscent of Van Gogh or Cezanne.

Throughout the 1950s Thomas was a regular exhibitor at the Royal Academy, but by the next decade commercial galleries began to favour abstraction. She did not change her style and so her work never gained the critical recognition which it merited. She and her husband continued to run a summer school for artists at Slindon in west Sussex. She died from breast cancer in 1995.

Six oil paintings by this artist are in the permanent collection of Falmouth Art Gallery, and were photographed for the Public Catalogue Foundation's 2007 survey. A further three of her oil paintings can be found in the collection of Penlee House Museum.