His father, Prof. James Dornan, is an obstetrician, who often lectures at QUB. He was a teenager when his mother died. He attended and boarded at Methodist College, where he excelled on the rugby pitch as well as in the drama department. While at MCB, he met David Alexander, who would become his songwriting partner in 2002, when they founded their two-piece Irish folk outfit, Sons of Jim. Dornan has been quoted as saying that if he ever entered the music business again, he would do it very differently. The boys were young, naïve, and under-prepared. Their record label, "Doorstep Records", was named after a sandwich shop in Belfast.
On the set of an Asprey photoshoot in 2003, Dornan met English actress Keira Knightley, who introduced him to her agent at PFD. The pair dated for three years. In 2006 he appeared in the Sofia Coppola film, Marie Antoinette. He used to perform in the folk band Sons of Jim, but the band disbanded in 2008. He has modelled for Calvin Klein (with both Kate Moss and Eva Mendes), Dior, Aquascutum and Armani, among many other brands.
Dornan appeared in the drama film Shadows in the Sun in 2009, which starred three-time Academy Award nominee Jean Simmons. In 2008, he played the lead role of Ed in the Hammer Horror production Beyond the Rave.
In 2011, he began playing the role of Sheriff Graham in the fantasy-adventure ABC television series, Once Upon a Time, that premiered on Sunday, October 23, 2011. A regular castmember for the first seven episodes of season 1, Dornan was eventually killed off from the series, shortly after his character's fairytale counterpart - the Huntsman - was revealed.