Smullen was born in County Offaly and was introduced to riding and racing by his brother Sean, who worked for trainer Joanna Morgan.

[1] Smullen spent two years as a stable jockey to John Oxx, winning his first Group 1 race, the Moyglare Stud Stakes, on Tarascon for trainer Tommy Stack at the Curragh in 1997.

[1] Smullen's first Classic victory came in the Irish St. Leger in 2001 on Vinnie Roe for Dermot Weld.

[1] In May 2018 Smullen announced his retirement from race riding on medical advice, having undergone chemotherapy and surgery for pancreatic cancer the previous year.

[3] In September 2019 Smullen organised a legends race at the Curragh to raise funds for pancreatic cancer trials and research.