Justice Horan was admitted in the Supreme Court of Victoria in 1992.  After practising as a solicitor and as a government lawyer, he commenced practice at the Victorian Bar in 2000, and was appointed senior counsel in 2015.  Justice Horan was appointed to the Federal Court on 5 September 2023.

In 1990, Justice Horan graduated from the University of Melbourne with a Bachelor of Arts and a Bachelor of Laws (Honours). In 1992, Justice Horan completed his articles and worked as a solicitor at Arthur Robinson & Hedderwicks. In 1993, Justice Horan was an Associate to then Justice of the High Court of Australia, the Hon Sir Francis Gerard Brennan AC KBE QC. From 1994 to 2000, Justice Horan worked for the Commonwealth Attorney-General's Department and the Australian Government Solicitor, including in the Office of General Counsel and as Counsel Assisting the Solicitor-General.

At the Bar Justice Horan specialised in administrative law, constitutional law, migration, revenue, native title, and commercial and equity matters.