In his most critically acclaimed period, he became an indelible part of British wartime cinema with starring roles in Saloon Bar (1940), The Next of Kin (1942), Went the Day Well?

In the postwar era, Johns worked regularly as a character actor at Ealing Studios with roles in They Knew Mr. Knight (1946), The Captive Heart (1946), Captain Boycott (1947), Easy Money (1948), and Scrooge (1951).

[1] He attended Llandovery College,[2] an independent boarding school in South Wales,[3] following the graduation of his brother Howard Johns, later rector of Pusey and Weston-on-the-Green.

[9] Johns made his stage debut while he and his first wife, Alyce Steele-Wareham, were touring the British dominions of South Africa, Australia and New Zealand in 1923.

Following this, he played the Magistrate's Clerk in Walter Hackett's Hyde Park Corner from 5 October 1934 to 11 April 1935 at the Apollo Theatre in London, the same year as the eponymous film.

[11] Johns made his screen debut in 1934 as the reporter in Ben Travers' comedy thriller Lady in Danger, going on to play Hemp in David MacDonald's 1937 crime film The Last Curtain, Sir Wilfred Lucas in the 1938 TV Movie[clarification needed] adaptation of Jane Austen's Pride and Prejudice, and Percival Clicker in Oswald Mitchell's 1938 comedy film Almost a Gentleman.

Johns avoided conscription due to his age, and thus began his career in various roles, though most often as the quirky yet dignified "frightened men" described by Adam Benedick.

Commenting on his role as the fearful architect Walter Craig in the 1945 mystery film Dead of Night, The Independent's Adam Benedick describes his approach as having a "masterly touch".

[21] His supporting roles in this era included playing Ernest Bennett in Ralph Thomas' romantic comedy film Helter Skelter, and Bob Cratchit in Brian Desmond Hurst's 1951 Christmas fantasy drama film adaptation of Charles Dickens's A Christmas Carol, with Alastair Sim as the cantankerous title character and miser.

In 1956, Johns was given the lead role of J. Philimore Sparkes in six episodes of the television series New Ramps For Old, in which he was cast alongside Harry H. Corbett and Colin Tapley, who played Kegworthy and Detective Inspector Welsh respectively.

He is remembered for standout roles as Arthur Charles Parfitt and Edward Lumsden in five episodes of the courtroom drama television series Crown Court alongside his grandson, actor Gareth Forwood, from 1973 to 1975.

In 1987, Johns appeared as a contributor in the documentary The Cavalry of the Clouds, produced by British regional commercial television station HTV West.

His first wife was concert pianist Alyce Maude Steele-Wareham, whom he married on 17 November 1922 in Saint Giles, London, and with whom he had his only child, the actress Glynis Johns, while on tour in Pretoria, South Africa.