The only daughter of the artist Norman GARSTIN and Louisa Jones Garstin (known as 'Dochie'), Alethea was born in Penzance and the only one of the three siblings to become a visual artist (the others were writers like their father).  Both Alethea and her brothers Crosbie and Denis, however, are listed by Iris Green in her study of the pupils of the FORBES SCHOOL, though no dates are listed.

Alethea had her first painting accepted by RA in 1912, and she exhibited regularly thereafter. She painted en plein air, subjects taken mainly from her many travels, as well as Cornish village life. She travelled extensively to destinations including Ireland, Belgium, France, Italy, Morocco, Kenya and Australia. She was a friend to many artists and was well-loved by all, accompanying Dod PROCTER on some painting holidays, but finding the experience quite exhausting and sometimes embarrassing due to Dod's drinking habits.

In 1960 she moved to Zennor where she spent the rest of her life, and died shortly before a major exhibition of her and her father's work opened in St Ives in 1978. The general impression is that her work has so far been neglected and that critical acclaim is overdue. Her portrait of Norman GARSTIN is located at Plymouth City Museum. This was illustrated in the Falmouth Art Gallery catalogue for the Women Artists in Cornwall exhibition in 1996, along with seven other oils by the artist.