Francis Johnston first trained under the architect Thomas Cooley. Two early projects were the completion of Rokeby Hall and Ballymakenny Church, Co.Louth, to the designs of Cooley.At a time of huge rebuilding in Georgian Dublin, Johnston was one of the architects responsible for Sackville Street (now O'Connell Street). The great Pillar (destroyed by a bomb in 1966) and General Post Office (GPO) were designed to harmonise with each other in the street adding grandeur and elegance to the boulevard.His work is interesting from an architectural point of view, in that it spans both Neo-Classical and Neo-Gothic styles. His Chapel Royal in Dublin Castle is a fine example of an early Gothic revival church in Dublin. On this project, he worked closely with George Stapleton. The chapel proved a seminal building for later Gothic revival architects in Ireland. Among his other most notable projects were the construction of St.George's Church on the North side of Dublin, overseeing the conversion of Parliament House into the Bank Of Ireland and the construction of the Tudor Gothic gateway to the Royal Hospital, Kilmainham. In 1813 he began to work on Richmond Gaol as a prison to relieve the pressure on Newgate Prison, Dublin, which had been designed by his teacher Thomas Cooley.