Johnson was born in Blennerville, County Kerry, and studied law at University College Dublin (UCD).
He practised on the South Western Circuit (Kerry, Limerick and Clare) until he became Senior Counsel in 1977.
The Government of Ireland nominated Justice Nicholas Kearns on 7 October 2009 to replace him as President of the High Court.
After his retirement as president and a judge, he gave an interview to the media in which he suggested the legislature should revisit the issue of whether the death penalty should be allowed as punishment for particularly serious crimes, such as murder committed in the course of armed robbery.
[2] Mayor of Limerick Kevin Kiely supported the proposal as a response to the continuing increase in murders in Ireland.