[1][4] He was appointed as a High Court judge in Belfast, replacing Lord Justice MacDermott, on the latter's retirement.
[4] Gillen was sworn in before the then Lord Chief Justice, Sir Robert Carswell, on 6 January 1999.
[5] In 2016, an off-duty police officer who was representing himself in a house repossession case against Santander that Gillen was hearing tried to arrest him.
[6] The police officer was nevertheless sentenced, by Lord Chief Justice Sir Declan Morgan, to three months jail for contempt of court, with the possibility of release after 28 days if he apologised.
[7][8] From 2015 to 2017, he conducted a review of Civil and Family Justice in Northern Ireland.