Johnston grew up in Prescot, Lancashire and attended Whiston Infants School, Eccleston Park Junior School and Prescot & Huyton Grammar School for Girls - leaving at the age of seventeen after only one year of her "A" level course. She worked as a tax inspector before enrolling at the Webber Douglas Academy of Dramatic Art in London - at the age of twenty-one. She was awarded the OBE in the 2009 Queen's Birthday Honours List for her services to Drama and to Charity. After her stint on Coronation Street, Johnston joined the cast of Downton Abbey for its fifth season.