He is portrayed by actor Ruben Santiago-Hudson.
He trained Beckett as a detective and considers her his finest protege.
In roughly 2006, he was part of the investigation into the crimes of the serial killer known as 3XK, because he killed in threes. After taking 6 victims, he disappeared, then returned in 3XK (episode 3.6), claiming two more victims before disappearing again. Captain Montgomery kept in touch with one of the victim's fathers, who trusts him because he was always straight with him.
He was close friends with the New York City District Attorney Lou Karnacki until Beckett, Ryan, and Esposito discovered during a murder investigation that Karnacki withheld critical evidence in a high-profile murder trial in order to keep happy a wealthy couple who promised to donate to his mayoral campaign. Karnacki promised to make Montgomery Police Commissioner when he became Mayor in exchange for keeping his wrongdoing secret, but Montgomery refused.
In Knockout, Beckett, Ryan, and Esposito discover that Montgomery knew all-along the real reason for Johanna Beckett's murder, but kept it a secret from Beckett in an effort to protect her and his family. When he was a rookie, Montgomery befriended Detective John Raglan, the detective who investigated Johanna's murder, and his partner, Gary McAllister, and participated when they kidnapped and ransomed mobsters to control crime. One of their kidnappings went wrong when Montgomery accidentally shot an undercover FBI agent. The murder was pinned on an innocent man, for whom Johanna Beckett tried to build an appeal case. She and some associates were killed by a man named Dick Coonan, who was hired by Hal Lockwood, when she began researching the crime. In exchange for letting Beckett and his family live, Lockwood made Montgomery agree to keep an eye on Beckett. Out of guilt for his mistake, he built a career as a fine cop and, when he met Beckett, decided to do all in his power to keep her safe as further penance. Beckett catches onto Lockwood's involvement and is able to arrest him, with Castle's help. However, the leader of the conspiracy breaks him out of prison to come after Montgomery, and come to see both him and Beckett as too much of a liability. Lockwood gives Montgomery a choice for his failure to keep Beckett quiet: hand Beckett over to him to be killed, or have his family murdered. Montgomery decides to lure Lockwood and his cronies into the open so he can kill them, but after he mails a stack of files to an old friend named Smith in an effort to ensure Beckett's protection. After Montgomery confesses everything to Beckett and he tells her of his plan, she forgives him and tearfully refuses to let him risk sacrificing himself. Montgomery orders Castle to take her away for her own protection, which he does, kicking and screaming. Montgomery succeeds in killing Lockwood, but dies in the effort.
Later, as Beckett and Castle delve further into her mother's murder, they learn that Montgomery made a tape recording of a meeting between himself and U.S. Senator William Bracken, the man behind the entire conspiracy, where he admits to his involvement in covering up the FBI agent's death, and threatens to kill Johanna Beckett. Beckett finds the tape hidden in a set of elephant figurines that used to be her mother's and that she kept on her desk in the precinct. Armed with this information, she is able to arrest Senator Bracken, finally bringing her mother's killer to justice.
- Has been married to his wife, Evelyn, for 30 years at the time of his death and has three children: two daughters and a son.
- Enjoys playing poker
- Dreams of buying a boat and taking it on fishing trips if he ever wins the lottery.
- According to Beckett, he announces his retirement frequently, only to change his mind: "He's the Brett Favre of the NYPD."