Nesbitt was born to a Northern Irish family, his father was a teacher. Nesbitt himself wanted to become a teacher as well, so he began a degree in French at the University of Ulster. He dropped out of school after a year when he decided to become an actor. He graduated in 1987 and spent seven years performing in multiple plays. He made his film debut playing a talent agent in Hear My Song.
His notable roles include the title role of the BBC miniseries Jekyll, a modern-day sequel to Robert Louis Stevenson's novel The Strange Case of Dr. Jekyll and Mr. Hyde. He is also the host of a travel series touring around his native Ireland.
Nesbitt is married to actress Sonia Forbes-Adam, who has been cast as Belladonna (Took) Baggins (Bilbo Baggins' mother) in The Hobbit: The Desolation of Smaug. The couple have two daughters, Peggy and Mary, who likewise will appear in the film as Bard the Bowman's daughters.
- Nesbitt joked that, now that every member of his family belongs to the cast of The Desolation of Smaugh, they will henceforth be referring to themselves as "the von Nesbitts."
- One of Weta Workshop's makeup artists commented that Bofur's facial prosthetics were originally intended to be much more extensive, but Nesbitt had such an expressive face that these prosthetics were scaled down to leave most of his face uncovered.