Johnston may be best known as Barbara Royle in the BBC comedy series The Royle Family, appearing with her former on-screen husband in Brookside, Ricky Tomlinson, from the show's inception in September 1998 until it ended at Christmas 2000. She also appeared in a one-off special which aired in October 2006. From 2000-11, she starred in the television series Waking the Dead, in which she played the role of psychological profiler Grace Foley, alongside Trevor Eve.