 The designs of Galtrim House, Co. Meath (c. 1802) and Ballynegall House, Co. Westmeath, built in 1808, are attributed to Johnston.In 1813 he began work on Richmond Gaol as a prison to relieve the pressure on Newgate Prison, Dublin, which had been designed by his teacher Thomas Cooley.He played a major role in designing Nelson's Pillar in Dublin, the construction of which was started in 1808.