Mrs Johnston is a National Trustee of the BBC for Northern Ireland and Pro-Chancellor at the Queen’s University, Belfast. Mrs Johnston has a number of property interests. Prior to this she was a shareholder and director of Variety Foods Ltd, a food service company supplying the Irish market.

She was previously a Council Member of the Northern Ireland Economic Council and a Board Member of the Labour Relations Agency and also worked in both economic development and the textiles sector. Mrs Johnston is also a Director of AIB UK plc.