Nash was born in London in 1893, the younger brother of Paul Nash. Educated at Wellington College, he exhibited from 1913, and fought in World War I from 1916 to 1918 with the Artists Rifles. He worked as an official war artist from 1918. From 1924 to 1929 he taught at the The Ruskin School of Drawing and Fine Art (Oxford), and from 1934 to 1940 taught at the Royal College of Art (London), working on wood engravings, lithographs, etc. He started World War II in the Observer Corps, moving to the Admiralty as an official war artist with the rank of Captain in the Royal Marines in 1940. He was promoted Acting Major in 1943 and relinquished his commission in November 1944. He died in 1977 in Colchester.
John Nash had formal art training but was encouraged by his brother to develop his abilities as a draughtsman. His early work was in watercolour and included biblical scenes, comic drawings and landscapes. A joint exhibition with Paul at the Dorien Leigh Gallery, London, in 1913 was successful, and John was invited to become a founder-member of the London Group in 1914 and to join Robert Bevan's Cumberland Market Group in 1915.
He began painting in oils with the encouragement of Harold Gilman, whose meticulous craftsmanship influenced his finest landscapes such as The Cornfield (1918; London, Tate).The Cornfield was the first painting Nash completed that did not depict the theme of war. The picture with its ordered view of the landscape and geometric treatment of the corn stooks prefigures his brother Paul's Equivalents for the Megaliths. John said that he and Paul used to paint for their own pleasure only after six o'clock, when their work as war artists was over for the day. Hence the long shadows cast by the evening sun across the middle of the painting.
His most famous painting is Over the Top (oil on canvas, 79.4 x 107.3 cm), now hanging in the Imperial War Museum, London. It is a celebrated image of the attack during which the 1st Battalion Artists Rifles left their trenches and pushed towards Marcoing near Cambrai. Of the eighty men, sixty-eight were killed or wounded during the first few minutes. Nash was one of the twelve spared by the shellfire and painted this picture three months later.
After World War I his efforts went into painting mainly landscapes. Emotions, however, concerning the war continued to linger for many years and this was depicted in his landscape painting. This is particularly evident in The Moat, Grange Farm, Kimble, oil on canvas, exhibited in 1922. In this brooding landscape the trees and their tendril-like branches envelope the entire picture plane.The dark subtle colours and evening light give the painting a claustrophobic atmosphere. This painting, completed a few years after the war, is characterised by a sense of bleak desolation that suggests the profound introspection that for many followed the devastation of the war. Although he had a great love of nature he often used natural subjects to convey powerful and sensitive thoughts concerning the human condition.