He is first heard of during the Wars of the Roses, when he was acting as legal adviser to James Butler, 5th Earl of Ormonde, who was a staunch supporter of the House of Lancaster.
Ormonde was executed by the rival dynasty, the House of York, after their decisive victory at the Battle of Towton in March 1461, and Bermingham himself was condemned to death as a traitor in 1462.
[citation needed] He became King's Serjeant in Ireland in 1463; the following year he was nominated as Chief Justice of the Irish Common Pleas but for unknown reasons did not take up that office.
Many years later Sir William Darcy, the Vice-Treasurer of Ireland, recalled that in 1482-3 he had been one of a number of students who read the leading English legal texts with John Estrete, Deputy to the Chief Baron of the Irish Exchequer, and studied Law French (necessary for pleading in Court), while during the holidays Bermingham taught them dancing and the harp (these activities were not simply for recreation but formed part of a young lawyer's education).
[6] Nicholas Sutton, Baron of the Court of Exchequer (Ireland), who died young in 1478, thought highly enough of him to leave his own family in Bermingham's care.