Born in Penge, London, she studied at the Slade School from 1912 to 1914, also in Karlsruhe, Germany. Her move to Cornwall in 1921 was to start working (briefly) with Stanhope FORBES. She was influenced by Algernon Mayow TALMAGE to paint in oil because he thought she was so bad at watercolours. She went on to develop an individual style of still life in the 1930s, and still later, under the influence of Ben NICHOLSON, this developed into an abstract style.
She married Henry Armstrong, a civil servant, and moved to Carbis Bay in 1921 whilst maintaining studio initially at 1 Piazza Studios, then at 9 Porthmeor Studios, St Ives. In their early days there, she and Henry acquired Rose Cottage, the former home of writers Edith and Havelock Ellis, which was to become their permanent residence.
At the March Show Day in St Ives (1924) she exhibited posters, book illustrations and a flower and fruit study. As Shearer Armstrong she exhibited regularly at the RA and also at the Royal Scottish Academy. Through her friendship with Mary PEARCE and her interest in the paintings of Bryan PEARCE she was to be the conduit for Pearce's first major show of work in 1959 (25 paintings) at the Scottish Gallery in Edinburgh, where he shared a three-person show with Armstrong and the St Ives artist Guy WORSDELL.
From 1926 she also exhibited at Newlyn, and in later years she gave art classes to the residents at St Teresa's Cheshire Home, Marazion. At this later stage she returned to a more traditional style of floral painting, of which there is an example in the permanent collection of the HYPATIA TRUST.