He was portrayed by Courtney Gains.
Nunn hired Warrick Tennyson, an explosives expert, to build and plant the bomb, while Nunn detonated it with a cellualr phone.
Twelve years later, he was hired to build another bomb, as part of a scheme by Dale Biederbeck, and plant it in the back of a car meant to be ridden by the Governor of California and his wife during his hometown's bicentennial parade.
Unknown to him, Nunn is also a pawn in a related scheme to frame Monk for murder and incapacitate Monk.
In "Monk Takes Manhatten", Warrick Tennyson tells Monk that he was hired to build the bomb which killed Trudy, by a six-fingered man. This six-fingered man detonated the bomb from a distance with a cellular phone.
Over two years pass without Monk getting any closer to finding the people responsible for killing his wife.
That is, until the Season 6 finale "Mr Monk is On the Run", when Monk and Natalie are called to the scene of an electronics store robbery. Several items have been stolen that combined, can be used to build a bomb, and a crowbar dropped by the thief bears fingerprints from a man with six fingers on his right hand. Monk believes this to be the same man who murdered Monk's wife, Trudy.
A note is also left behind which reads: "To Force Heaven Mars Shall Have A New Angel". When Monk decodes the message as saying "247 Marshall Avenue, Angel, California", he goes there alone, taking along his old police-issue pistol. There, he meets Frank Nunn, who recognizes Monk as "the cop with the wife." Frank Nunn shows no remorse and in rage, Monk lunges at him and the two engage in a violent brawl, during which Monk repeatedly kicks and punches Nunn before drawing his pistol and demanding to know who hired him. Before Nunn can answer, he is shot dead by a corrupt local sheriff, John Rollins.
Later though he appears to be some use as the police find papers in Frank Nunn's apartment connecting Trudy's murder with a mysterious figure known as "The Judge".