 He was the architect who assisted the 2nd Earl of Longford in turning Tullynally House of County Westmeath into Tullynally Castle, completing these works in 1803.In Swords, County Dublin, he designed the Old Borough School (now a public house) on the main street, completed 1809, and designed Saint Columba's Church of Ireland church, completed 1818.Johnston designed the Courthouse in Duleek, County Meath.