The Royle Family actress will play the feisty mother of Rover's Return landlady Stella from September.
Johnston explained to The Sun: "My son lives in Manchester and he's just had my first grandchild.
"So it made sense to be working where the baby is."
Her new character Gloria will upset Weatherfield locals when she moves into the Rover's next month.
"Gloria doesn't endear herself to everyone - she's a real Coronation Street matriarch," added the 68-year-old.
Johnston's entrance story was changed after her original scenes were cut due to illness in May.