In 1948, he bought a one-way ticket to Britain and learned his craft working in repertory theatre at Southend-on-Sea for almost five years. He began to appear in British films from the mid-1950s, including a role in the classic Ealing comedy The Ladykillers (1955). He ran a small hotel in London during the 1950s, and was a member of the English Stage Company at the Royal Court Theatre during the Angry Young Men period when new playwrights, including John Osborne, introduced new themes to British theatre. His most famous character, Barlow, was noted for his hard edges, owing much to the changes in characterisation pioneered at the Royal Court.

He married Nanette Ryder in 1955; they had four children. He ran the small hotel in St Martins Lane called the St Martins Hotel; it was managed by Elizabeth Kissick-Jones, formerly Hartnell, who was the aunt of his wife Nanette. The hotel was very popular with actors and he ran it until 1976.

He was also the author of the children's book Gumphlumph; in the mid-1960s, at the height of his fame as Barlow, he read it on the children's television series Jackanory. Gumphlumph would be revived, again with Johns narrating, for the TV-am children's programme Rub-A-Dub-Tub in the 1980s.

In 1962 he won the part of Barlow in Z-Cars and soon became one of the most familiar and popular faces on British television. During the long run (1962–1965) of Z-Cars, he transferred his character to the spin-off series, Softly, Softly (1966–1969), and later Softly, Softly: Taskforce (1969–1972). He also played the voice of the mysterious "Guvner" in The Great St Trinian's Train Robbery (1966).

He was the subject of This Is Your Life in October 1963 when he was surprised by Eamonn Andrews at BBC Television Centre.

His many stage credits include Daddy Warbucks in the original West End run of Annie - he can be heard on the original London cast album - and the Ghost of Christmas Present in the original Birmingham cast of the stage adaptation of the film musical Scrooge (1970), on the recording of which he can also be heard. His guest appearances on TV include The Avengers, Department S, Neverwhere, the Doctor Who serial Four to Doomsday (1982) and the Blake's 7 episode "Games". He had a prominent role as Calpurnius Piso in the BBC's acclaimed adaptation of Robert Graves' I, Claudius (1976); he played Magwitch in the BBC's 1981 adaptation of Dickens' Great Expectations, and the jailer in The Secret Life of Albie Sachs. Johns appeared in the 1984 pop video for 'Young at Heart' recorded by The Bluebells. With him were veteran Scottish Actress Molly Weir and Scots singer/actress Clare Grogan. In 1993, Johns appeared in the BBC period drama Scarlet and Black alongside a young Ewan McGregor and Rachel Weisz.

In 1973 Johns was named BBC TV Personality of the Year by the Variety Club of Great Britain. He portrayed the apartheid-supporting Namib mine superintendent Mr. Zimmerman in two episodes of the 1985 mini-series Master of the Game.

Johns later appeared in the much-maligned Ken Russell films Salome's Last Dance and The Lair of the White Worm (both 1988), followed by the title-character in the mid-1980s Channel 4 series Brond.

After several years of poor health, Johns died from heart disease in 2002, aged 76.