Born in Hastings, Sussex, John was the third son of a clergyman, and his early years were spent in West Dean, near Goodwood. He studied law at St John's College, Oxford and then St John's Wood School of Art in London before and after the 1914-18 war, enlisting in the Royal Field Artillery during the conflict.

During WWII he was an Official War Artist. After parting from his first wife, Benita (nee Jaeger) he spent the war years with his new partner Veronica Sibthorp, in Dunmow, Essex. 

Armstrong had never previously visited Cornwall when, in 1945, in his fifties, he and Veronica moved to Oriental Cottage. Despite a turbulent relationship, they married in 1953. He was already an artist of some considerable stature, having been a member of Paul Nash's Unit One (1934), Head of Costume & Design at Alexander Korda's film studios during the 1930s, and having created the powerful 'V for Victory' cover design for the winning Labour Party 1945 election manifesto. His famous 1938 solo show at the Lefevre Gallery led to his being acclaimed as 'England's leading surrealist'. However, his work, which increasingly expressed his fear of the annihilation of humanity in the face of global conflict, did not find favour with Sir Kenneth Clark and the Arts Council, so his move to Lamorna provided a fresh start away from the artistic mainstream.

In Lamorna he remained engaged in painting surrealist murals for the London stage which he had been successful with, both for stage and film. Special friends were Elsa Lanchester and Charles Laughton. The paintings he embarked on in Cornwall did not focus on the local landscape or inhabitants. But the gigantic vegetation of the Lamorna valley provided a stimulus for his fascination for form in nature and what followed was a series of symbolic works which combined natural shapes with those of classical figures.

In 1951 Armstrong's work was hung for the first time at the Royal Academy, and the same year he achieved further success at the Lefevre Gallery.

In Cornwall he joined with John TUNNARD, Peter LANYON and others irritated by the division of art into categories of figurative and non-figurative by the Penwith Society, and began to show his work at NAG, becoming a re-invigorating force within the NSA which they all joined. Armstrong served on the organising Committee of the Newlyn Society of Artists until 1955 when he resigned from both the Society and the Committee on his departure from Cornwall.

In the early 1950s when Armstrong had been commissioned to paint the ceiling mural for the Bristol Council House, he did this work on canvas in the lower front room of NAG (where administration offices are today) which he contracted to rent for a year (1954).  The canvas was so large that it had to be wound around three giant rollers, from which it was unwound as needed and painted a section at a time. An assistant for the year was a young woman named Mary COLLETT, who has continued as a painter in her own right. Later the canvas was 'marouflayed' (stuck) to the Council House ceiling where it remains.

In 1955 he returned to London, divorcing from Sibthorpe, and marrying his third wife, Annette. Armstrong was made an Associate of the RA in 1966. He also painted murals for the Royal Marsden Hospital. Precise but muted in colour, his symbolist qualities are keynotes of his style, and always his work was abstract, upon occasion surrealist.

During the early 1960s Armstrong was diagnosed with Parkinson's disease. Annette and their daughter Catherine cared for him at the family home in Putney, where he continued to paint until shortly before his death in 1973.

In 1989, a painting by John Armstrong, from the Permanent Collection of NAG, Veronica as Harlequin, was chosen for the Cornwall County Council Exhibition, 'A Century of Art in Cornwall 1889-1989' in which 140 paintings executed in Cornwall over the 100 year period were shown. The painting had been given to NAG by Frankie FREETH, another artist who had served with Armstrong on the NAG committee, and a close friend and partner to his ex-wife Veronica.