From a family with no racing connections, Heffernan was introduced to the sport when he took a summer holiday job with the National Hunt trainer Arthur Moore.

He began his racing career as an apprentice jockey for P J Finn and rode his first winner on 10 August 1988 at the age of sixteen.

[2] When Finn retired he moved to the yard of Jim Bolger and shared the Irish champion apprentices title in 1994.

[1] In 2019, Heffernan rode Anthony Van Dyck under trainer Aidan O'Brien to win his first Derby victory.

[4] On the eve of the 2024 Irish flat season, Heffernan announced he was leaving his permanent position at Ballydoyle for a new career as a freelance jockey, although both parties stressed there had been no falling out and said they expected to work together again in future.