Leland (played by Ray Wise) is an attorney, with his primary client being local businessman Ben Horne. He is well-known and respected in the town of Twin Peaks. Together with his wife Sarah and his daughter Laura, his family seems to be perfect. When Laura is murdered, however, Leland’s psychological foundations begin to crumble, and it is gradually revealed that extreme dysfunction, including incest, lies beneath the Palmer family's idyllic surface.
In the pilot episode of the series, Leland is called into the morgue to identify Laura's body. The grief pushes him over the edge; he throws himself into the grave at Laura's funeral and has to be dragged out, and for the next few weeks he manically asks people - even complete strangers - to dance with him as Laura had, all while sobbing hysterically.
When Jacques Renault is arrested as a suspect in Laura's murder, Leland murders him by suffocating him with a pillow. After killing Jacques, Leland's hair turns white overnight. Leland is arrested, but released on a personal recognizance bond pending trial due to personal assurances made by Sheriff Harry S. Truman that he does not pose any danger to the community. Leland's behavior at times appears abnormal - manically happy at times, exhibited by singing the 1940s popular nonsense song "Mairzy Doats", before crashing into grief. The arrival of his niece-in-law Madeline "Maddy" Ferguson, who looks almost exactly like Laura (both roles are played by Sheryl Lee), is of great help to him in learning how to deal with his daughter’s death.
It is eventually revealed that, as a boy, Leland met a strange neighbor who turned out to be a demonic entity named BOB, who would possess him throughout his life. It is also implied that BOB molested him as a child. It is difficult to tell when and if Leland is truly in control of himself and when he is being possessed by BOB. It is implied that Leland killed Jacques Renault on his own – not under BOB's control – and that Leland is lost from that point: he is no longer able to stop BOB and is completely taken over, physically manifested by the change of hair color and a dramatic change in behavior. Finally, BOB takes control of Leland and murders Maddy. The changes in Leland from this point help Special Agent Dale Cooper to finally identify him as BOB's "host".
Leland is eventually exposed as the murderer of both his daughter and his niece Maddy and is arrested by Cooper and Truman. He later dies in custody following a bizarre interrogation on the night of his arrest. BOB takes control of Leland and forces him to run headfirst into the metal door of an interrogation room. At the moment of his death, Leland remembers the crimes he committed under BOB's control, and cries out in agony. After Cooper recites the last rites from the Tibetan Book of the Dead, Leland sees a vision of his daughter Laura and dies. Later on, Cooper explains to the widowed Sarah that he believes the vision was of Laura forgiving Leland and welcoming him into the afterlife.
The prequel film Fire Walk With Me, which depicts the last week of Laura's life, reveals that Bob had begun raping her when she was 12 years old while possessing Leland. The abuse drives Laura into a life of drug addiction and promiscuity, and she begins working as a call girl. She often turns tricks together with Theresa Banks - whom Leland frequently visits while away on business. He arranges a "party" with Banks and two other girls, only to find out that the two girls are Laura and her friend Ronette Pulaski. He pays Banks and leaves, but comes back later that night and beats her to death.
A year later, he sees Laura having group sex with Pulaski, Renault and Leo Johnson; enraged, BOB uses Leland to attack, beat and rape both girls, before killing Laura, even as Leland begs BOB not to make him do it. He wraps Laura's body in plastic and dumps it in a river. He then appears in the Black Lodge, where he meets The Man From Another Place and MIKE, BOB's former partner in murder.
When Twin Peaks was revived in 2017, Ray Wise confirmed that he would appear in the new series, once again portraying Leland Palmer. In the second episode of the renewed series, Leland appears in the Black Lodge imploring Cooper to "find Laura". In Part 18, he also repeats his instructions to Cooper when he returns to the Black Lodge. Due to time alterations following Part 17, it is said that Leland committed suicide on February 24, 1990, one year after his daughters disappearance (she was never confirmed to have been murdered in the changed timeline).