Daughter of George and Mary Harris, Mary married Robert Thomas, a Welshman, at Southampton in 1818. They had seven children. In June 1836 Robert Thomas, with his friend and partner George Stevenson, published in London the first edition of the South Australian Gazette and Colonial Register. Their eldest son, Robert George Thomas, came to SA in 1836 as one of Colonel William Light's staff on the Cygnet, and the rest of the family, Francis (eighteen), William (fifteen), Mary (thirteen) and Helen (eleven) came soon afterwards on the Africaine, arriving in November 1836. At this stage Robert was 55 and Mary Thomas was 49. Robert (Snr) published Governor Hindmarsh's Proclamation of the Province, 28 December 1836.
The family lived first in tents and a brush hut at Glenelg, then in March 1837 moved to Hindley Street where they established a general store, stationery and printing business, and in June 1837 published the first colonial edition of the South Australian Gazette and Colonial Register. The Stanhope Press they used is on display at the State Library of South Australia. Robert loved living in South Australia, but Mary always hoped to return to England. She kept a journal between 1836 - 1841, which was later published, along with her letters to her brother George in Southampton, by her grandson Evan Kyffin Thomas. It includes some of her poetry. She died fifteen years after her husband, at their English style cottage "Rhantregwnwyn Cottage" at the western end of Hindley Street, and was buried in the West Terrace Cemetery.
