Spotted while acting in a chapel pageant competition, after an audition John trained at the London Academy of Music and Dramatic Art, graduating in 1950.

On Radio 4, she appeared on Linda Smith's A Brief History of Timewasting and played Mrs Stone, the school secretary, in the original ten series of King Street Junior from 1985 to 1998.

In a career of over 50 years, she appeared in television programmes such as: Coronation Street, Dixon of Dock Green, Z-Cars, Doctor Who, Little Britain, Emmerdale, Last of the Summer Wine, Crossroads, Gavin & Stacey and The Mighty Boosh.

[8] In September 2009, John appeared in the graphic short film Cow by director Peter Watkins Hughes warning of the dangers of texting while driving.

[11] In March 2010, she appeared in the BBC One Wales programme Margaret John – National Treasure, which featured clips from the last 50 years of television and special guest interviews with, among others, Ruth Jones, Eve Myles and Joanna Page.