Alethea was born in Penzance, the daughter of acclaimed painter Norman Garstin. She began painting when she was 16, and aged 18 she had her first painting accepted by the Royal Academy. She received no formal tuition as a painter, but was probably taught largely by her father, who also taught artists such as Harold Harvey.

Alethea became influenced by the Fauvist painters and her mature work, which was usually on a small scale, used a limited palette and broken, loose brushwork. She nearly always painted en plein air, directly onto board, with a toned back-ground, using square hog-hair brushes. Her confident handling of paint and lively compositions prompted Patrick Heron to describe her in 1978 as ‘England’s leading Impressionist’ and her work as ‘as good as Vuillard’. Her subjects included many of the scenes and characters she witnessed on her travels through Ireland, Belgium, Italy, France and to the Isles of Scilly as well as friends and views closer to her Penwith home.