She is known for portraying Sheila Grant in the Channel 4 soap opera Brookside (1982–1990), Barbara Royle in the BBC comedy The Royle Family (1998–2012), Grace Foley in the BBC drama Waking the Dead (2000–2011), Gloria Price in the ITV soap opera Coronation Street (2012–2014) and Miss Denker in the ITV drama Downton Abbey (2014–2015).
Since then she has appeared in many drama series and films, including Inspector Morse, Hetty Wainthropp Investigates, Brassed Off and My Uncle Silas.
In 1992, Johnston appeared in the three-part award-winning drama Goodbye Cruel World, in which she portrayed a woman coming to terms with a muscle-wasting illness.
From 2000 to 2011, she starred in the television series Waking the Dead, in which she played the role of psychological profiler Grace Foley, alongside Trevor Eve.
[7] She starred in Jennifer Saunders's comedy drama Jam & Jerusalem on BBC One, alongside Joanna Lumley, Maggie Steed and David Mitchell.
She shared a role with Billie Piper in the television adaptation of A Passionate Woman which aired on BBC One on 11 April 2010.
[9] On 2 April 2012, Coronation Street series producer Phil Collinson announced Johnston had joined the soap opera as Gloria, the mother of Stella Price (played by Michelle Collins).
[21] In 2024, Johnston was announced to appear as a contestant in the "New Year Treat 2025" special of Taskmaster, alongside David James, Hannah Fry, Martin Lewis, and Melanie Blatt.
In 1989, Johnston, assisted by Lesley Thomson, published her first book, a memoir titled Hold on to the Messy Times.
She is distantly related to horse trainer Mark Prescott, via her maternal line, as discovered by ITV's DNA Journey; aired 16 Oct 2024.