John left drama school in 1950. She starred in the BBC's 1960 adaptation of Richard Llewellyn's novel How Green Was My Valley as Bronwen and had a brief spell in soap opera Coronation Street in 1965.
She starred in four episodes of Z Cars in 1967 and appeared as Megan Jones in three episodes of the Doctor Who adventure Fury From The Deep in 1968.
Other roles in the late 1960s and early 1970s included appearances in BBC dramas The First Lady, Softly Softly, Dixon Of Dock Green and also Emmerdale Farm.
The late 1970s saw John star as Marian Owen in another soap opera, Crossroads (famous for its wobbly sets), and from 1984 she starred in medical drama The District Nurse as Gwen Harris alongside fellow Welsh actor Nerys Hughes.
Highlights in the 1990s include roles in Sherlock Holmes dramas Sherlock Holmes and the Leading Lady and Incident At Victoria Falls, Lovejoy and the Ruth Rendell Mysteries.
She played the part of Elsie 'Mam' Hepplewhite in BBC Wales comedy High Hopes between 2002 and 2008, starring alongside Robert Blythe and Boyd Clack.
In 2003 she starred in bingo-related comedy Eyes Down, with Paul O'Grady and Rosie Cavaliero. Appearances in The Mighty Boosh and Little Britain followed before she once again starred in Doctor Who, The Idiot's Lantern in 2006 - 39 years following her initial role in the drama.
She starred in David Schwimmer's British film comedy Run Fatboy Run (2007) and enjoyed recent appearances in medical dramas Casualty and Doctors, and the Welsh-set BBC drama Framed with Trevor Eve and Eve Myles.
In May 2009 John received the Bafta Cymru Lifetime Achievement award.
After starring in the hugely successful Gavin & Stacey as Doris, John had a role in spoof Valleys film comedy A Bit Of Tom Jones and appeared in the second series of BBC Three drama Being Human.
In August 2010 she joined the cast of Calendar Girls in the Wales Millennium Centre in Cardiff.
Margaret John died in February 2011, aged 84.
From Maria to the Phantom, how Welsh stars have brought song to the stage.