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. It was built circa 1838 as a sessions house for the Meath Grand Jury, commissioned by John Trotter. It was converted to a library and environmental offices in 1960. He lived for many years in his house at 64 Eccles Street, where he kept a large collection of curiosities. He died in 1829 and was buried in St. George's churchyard, which was attached to a temporary church on Whitworth Road (later taken over by the Whitworth Hospital).