Justin qualified as a dentist from the University of Wales College of Medicine in 1989. Following hospital posts and a position as registrar in restorative dentistry, Justin got his Fellowship in Dental Surgery and was accepted on the postgraduate orthodontic masters course in Cardiff in 1993.
After qualifying in 1996, Justin took up a specialist orthodontic senior registrar post in Sydney, Australia. He became acting head of department, published academic articles and, following a two-year period as an associate dentist in Bristol, Justin returned to Wales. There he joined the North Cardiff Orthodontic Centre which provides orthodontic treatment to under-18s on the NHS.
In 2016 Justin co-founded Crocodile to offer more choice and convenience to children and young people in need of orthodontic care, and he also co-founded Embrace, an adult orthodontic clinic.