Although Martin is first mentioned in an episode of the sitcom Cheers, his life history is mainly expanded on in Frasier. In an eighth season Cheers episode ("Two Girls For Every Boyd"), Frasier claims that his father is dead and was a scientist in life. These comments are elaborated on Frasier as the result of estrangement between Martin and Frasier at the time. (Mahoney also appeared in Cheers as a piano player/jingle writer.)
The son of a police officer, Martin was born in Seattle in 1932 and is a lifelong resident of the city. He may be of Irish descent, as he and Frasier once sing the song 'Danny Boy' while bonding over a beer. At the age of 19, Martin joined the U.S. Army, and saw combat in the Korean War. After returning home Martin joined the Seattle Police Department, and would remain in this capacity until being forced to retire due to injury in his late fifties. He served in various roles including several years on mounted patrol, and attained the position of Homicide Detective. He was married to Hester Rose Crane, a forensic psychiatrist whom he met in 1952 at a murder scene. They remained married until her death from an unspecified illness in 1987, some six years before Martin moves in with Frasier.
Hester rejected his first marriage proposal, but the two married when she became pregnant with Frasier. The marriage was mostly a happy and loving one, but there were numerous issues; in particular, Hester had an affair with a family friend following a particularly difficult period in the marriage. Martin blamed himself for his wife's infidelity, which occurred about 30 years before he moves in with Frasier, and initially tells his sons that he had been the adulterer. The revelation brings Martin and Frasier, who after he learns the truth confesses that Lilith also cheated on him, closer together.
Martin was greatly affected by Hester's death, an event that seems to have increased his estrangement from Niles and Frasier, and certainly contributed to his reluctance to engage in relationships with other women. For much of their adult lives, Martin, Frasier, and Niles seem to have had a rather distant and estranged relationship with one another.
Martin has one sibling, a brother named Walt (who is married to a Greek woman named Zora), and one nephew, Walt and Zora's son Nikos. Martin and Walt are estranged, thanks to Zora's hostility toward Frasier's meddling in Nikos' life(encouraging him to follow his dream of becoming a professional juggler, rather than the more reputable profession his domineering mother wanted for him). While they reconcile in the episode "Beware of the Greeks" (season 5 of Frasier), it is unclear how long the reconciliation lasts, and Walt appears in only one episode. There is a discrepancy in the storyline, however, as Martin states earlier, "I never had a brother." Two years before the series began, Martin was looking forward to retirement when he was shot by a man named David Hicks in the line of duty after interrupting a robbery attempt in a convenience store. Niles visited him, but the two had drifted apart. Although he hoped to return to work in a few weeks, the injury left a bullet in his left hip, forcing him to walk with a cane, held in his right hand, for the rest of his life, and to retire from the police force.
His injury inhibits him from living alone, and forces him to accept Frasier's invitation to live with him upon Frasier's return to Seattle. Martin soon grows close to his physical therapist Daphne Moon, and steadily becomes closer to both his sons, despite their different tastes and personalities. Because both his sons share very little in common with him in terms of hobbies and personalities, he also grows to be very close to Frasier's producer Roz. He invites her to his poker games on numerous occasions, and responds warmly to her laid back, "one of the guys" personality. On one of his birthdays, she presents him with a six pack of beers, and this turns out to be the only gift he seems to genuinely enjoy that day. In many ways, because she shares so much of his own interests, he seems to view her as a good friend. In turn, Roz responds to his feelings.
Despite already being handicapped and retired from the police force, one of Martin's greatest achievements came in his sixties when he solved the notorious "Weeping Lotus" murder, a cold case that had confounded himself and the rest of the police force for decades.The resolution brought a long-awaited measure of satisfaction and peace of mind to Martin, who had made a vow to the prostitute victim's mother that he'd find and apprehend the killer, and was becoming concerned that she would die of old age before he did.
Romantically, Martin is reluctant to begin dating again, but while living with Frasier he has a number of relationships with various women. In at least one episode, he reveals that he is strongly attracted to East Asian women. Although he is never shown dating one during the show, he has fond memories of "dating" during his time as a soldier in Korea. By the end of the show, Martin reacquaints himself with Ronee Lawrence, a woman who, as a teenager, babysat Frasier and Niles. They become involved in a romance, which escalates into a marriage proposal. The ceremony takes place in the final episode of the series, establishing a new chapter in Martin's life.