Thel. Smith was the younger daughter of Stanley George Smith and his wife Elsie. The family moved to South Australia when Thel was twelve months old. Her father enlisted as a soldier in World War I and he was killed in the war in 1917. Her mother took the girls back to Western Australia to live, but by now her friends were in South Australia and she couldn't settle in the west, so they returned to Adelaide.
Thel went to the Flinders Street Practising School, then spent four years at the Methodist Ladies' College. She then got a half scholarship to Chartres' Business College. She later worked in the Children's Welfare Department, in the Boarding Out Office, finding homes for foster children. The man she wanted to marry was transferred to New York, but she felt she had to stay and look after her mother. Both vowed that they would never marry another; if they couldn't marry each other they would remain single. Both kept that promise.
Thel was a great reader. One of her earliest recollections is of having her light turned out when she wanted to read in bed. 'At least you can't turn out my thoughts', she said then. She began writing poetry at the age of six. She had her first poem published in the Sunday Mail, in the same issue as her sister Phyl, later to be an artist, had her first picture published.
In 1937 Thel won a writing competiton for the South Australian Rangers. She said she found romance and adventure everywhere around her, and she had a number of short stories and romances, as well as poems, published in women's pages and magazines. She had poems broadcast on radio 5AD, and these were published on request in 1940. Proceeds from the sale of the book were donated to war charities; the Fighting Forces Comforts Fund, the South Australian Division of the Red Cross and the Cheer-Up Hut. She wrote for the Girl Guides the lyrics for three songs, 'Bush Singing', 'Old Green Frog' and 'Camp Fire' which were set to music by Ariel Shearer, broadcast on 5AD, and recordings sent overseas. She also wrote sketches and plays for her Methodist Church at Vermont in the 1950s.
She retired from work at the age of 59, but continued writing until her health prevented her from doing so.