Sheriff Charles Mills was the local sheriff of Harper's Island and the father of Abby Mills.
Upon John Wakefield's arrival on the Island during the 1980s, looking for Sarah Mills, Charlie attempted to scare Wakefield away "for good" by having his deputies rough him up. Wakefield cut Deputy Cole Harkin in the process, and Charlie charged him with Attempted Murder of a Police Officer. Wakefield was sentenced to life in prison and served 17 years in a maximum security penitentiary.
Upon his release in 2001, Wakefield returned to the Island to seek revenge, committing a series of murders. Deputies Aiken and Harkin responded to a disturbance at the Marina, and were caught in an explosion. Aiken was killed, and Harkin was severely burned. Charlie arrived and attended to the injured Harkin, along with Henry Dunn. Later, Harbormaster Christopher Cullen, Kate Seaver, a secretary in the Sheriff's office, and Charlie's wife, Sarah Mills, were found hung in a large willow tree, thereafter named the "Tree of Woe."
Following the murder spree, Charlie tracked Wakefield and shot him, and Wakefield fell from the Eastern Bluff. Mills claimed to discover Wakefield's body days later at the bottom of the bluff, after the rest of the search party had moved on. In his grief over the death of his wife, Charlie sent his daughter, Abby, away from the Island to live with her grandmother.
In "Whap," after the two have had no contact in seven years, Charlie runs into Abby at the Cannery after Abby tries to break up a bar fight with a pool cue. When Charlie takes Abby back to the Candlewick Inn, he tries to reconnect with her, but Abby refuses to discuss their past. The next day, Charlie investigates an apparent break-in at the Maritime Museum. He notes that a head spade, gone from a display of whaling instruments, is the only item missing ("Crackle").
Charlie investigates the death of Kelly Seaver, who has apparently hanged herself in her house ("Ka-Blam"). Charlie informs Abby of Kelly's apparent suicide. Abby is shocked, citing how happy Kelly seemed the previous day, but she holds back telling him that she saw J.D. in Kelly's house. Charlie visits Shane at his house and asks if he has seen J.D. After the Sheriff leaves, Shane opens the back of his truck, where J.D. is tied up, and Shane accuses him of murdering Kelly. Charlie later returns and finds J.D. in a shed, bound and gagged, with a noose around his neck, his feet barely supported by a chair. Shane is arguing with Abby, and, in order to buy some time, Abby goes off on a tangent about how Wakefield killed Kelly. Sheriff Mills confronts Shane, and, in the scuffle, J.D. is knocked off the chair. Shane grabs J.D. as Abby runs to cut the rope holding him up, saving him. Shane is arrested. Later, as Doc Campbell informs him that Kelly was murdered, Charlie discovers a note in Kelly's house, presumably from the killer, taunting him, saying, "You found her, now find me."
In "Going to the Sheriff," Robin Matthews asks Sheriff Mills for help finding Brent Cyr who is missing. She also tells him that she has been attacked by the Cheshire Cat. Charlie is too busy investigating Kelly Seaver's tragic death to listen to Robin, and she urges members of the Harper's Globe website to call the Sheriff's station, asking for them to help Robin. Soon, one of the Sheriff's deputies reluctantly goes with Robin to Brent's parents' house.
In "Thwack," Abby is surprised when her father arrives at Jimmy Mance's house to meet him for breakfast, and she is floored when she discovers that they have become friends. They agree to go out to breakfast together, and Charlie asks Abby to drop by later to look through some of her mother's things. Abby promises to do so if she isn't too busy. Abby arrives at the house and attempts to call her father. She lets herself in after she leaves him a message. She goes through her old room, and then goes into the attic, where she discovers articles about the Wakefield murders and her mother's autopsy photos. After some more digging, she discovers articles about more recent murders that her father has marked as matching Wakefield's M.O. Charlie returns home, and Abby quickly returns everything to its place and runs downstairs. Charlie attempts to apologize for sending her away, but Abby hastily leaves. Later, as the wedding rehearsal begins at the Church, Charlie responds to a call from Deputy Patrick Lillis who is investigating a report that Reverend Fain has not been seen for several days, and Lillis tells Charlie that he has found the Reverend's hearing aid in the woods. As Charlie and Lillis search the woods, they find a stake with a line attached, running into a small pond. Lillis suggests that it may be a catfish line, and Charlie replies, "Your lips to God's ear." As Charlie pulls on the line, he discovers parts of Reverend Fain's dismembered body.
Charlie arrives at the Church immediately after Thomas Wellington is killed by a head spade falling from a booby-trapped chandelier ("Sploosh"). Charlie escorts everyone out of the Church and sends them back to the Candlewick. He begins an official murder investigation of the crime scene. Later, at the Inn, Abby, Henry, and J.D. confront Charlie with the picture of Wakefield found in his attic. J.D. resolves to dig up Wakefield's grave and Charlie locks him in the Candlewick's pantry. That night, as Abby and Henry dig up John Wakefield's grave, Charlie appears and shines a flashlight on them. He expresses surprise that it wasn't J.D. digging, and mentions that someone let J.D. out. Henry pries the lid from the coffin, revealing a human skeleton in rags. Charlie reiterates his belief in a Wakefield copycat to Abby as Henry covers the Wakefield grave. He further suggests that the killer is pretending to be Wakefield in an attempt to conceal his motive. Abby and Charlie reconcile, and he resolves to send everyone home in the morning.
In "Thrack, Splat, Sizzle," Henry and Sheriff Mills find Uncle Marty's cell phone in J.D.'s room, and they suspect him of committing the murders. As J.D. is tracked by bloodhounds, Henry tells Charlie about J.D.'s past suicide attempt, after which J.D. spent some time in a psychiatric facility. When the Sheriff's deputies close in on J.D., Henry runs ahead and tackles him, and J.D. is taken into police custody.
Cole Harkin tells Abby that J.D. is innocent and suggests she ask her father about the first time Wakefield came to the island. After Charlie returns to the Inn upon catching J.D., Abby confronts him about Harkin. Charlie tells Abby that Sarah had dated Wakefield in Seattle, and she came to the island to get away. Charlie confesses to Abby that the first time Wakefield came to the island he bumped up Wakefield's charges from assault to attempted murder of a police officer to send him away for life. After learning Harkin's whereabouts from J.D., Charlie goes to visit Harkin. On the way, he is badly injured in his thigh by one of Harkin's traps in the woods ("Gurgle").
Harkin takes Charlie to his cabin to care for his wound, where he shows Charlie the prison diary of John Wakefield. When Harkin goes to get help, he is hit by arrows on the porch and killed. He drops his lantern, starting a fire. After learning Charlie's whereabouts from Shane, Abby and Jimmy drive to Harkin's cabin and find Charlie inside. Charlie yells for Abby to save the bag containing the diary from the fire on the porch. Jimmy wants to take the injured Charlie to the clinic, but Charlie insists on going to the marina. Jimmy drives to the marina with Abby and Charlie. Charlie tells Abby to leave on Jimmy's boat and get to the mainland, but she refuses. Suddenly, Sully slams against the truck window, informing them that J.D. has escaped. Jimmy runs out, and Charlie gives Abby a second gun as she leaves.
In "Seep," Sheriff Mills is taken to the clinic, and Cal begins to treat his injured leg. Charlie tells Abby to keep Wakefield's prison diary safe. After Cal and Chloe become distracted, they discover that Charlie is missing, along with his oxygen tank. After Madison Allen is rescued from the underground tunnels, she asks Abby if she liked the game, stating the man who took her said Abby would like it. When Abby asks Madison who kidnapped her, she replies, "Your dad, Abby... You know, the Sheriff." Unable to comprehend Charlie's possible motives for kidnapping Madison, the group begins to suspect that he may be responsible for the murders ("Snap"). As Jimmy and Shane attempt to refuel Jimmy's boat in which to escape from the Island, Abby leads Henry and Trish into Charlie's attic, and they begins rifling through papers. Trish discovers maps of the Candlewick Inn's underground tunnel system, reinforcing her suspicions that Charlie is the murderer. As the group arrives at the marina, the gas dock explodes, destroying the marina. When the group comes under sniper fire, they retreat to the Cannery, where Shane reveals that he found the state police officers dead near the dock. He also tells the others that he saw Charlie's oxygen tank near the generator immediately prior to the explosion.
Later, a Sheriff's vehicle drives up to the Cannery and deposits an unconscious Jimmy in the parking lot. Abby and the others run out to help him, and Henry and Shane carry Jimmy inside. Taped to Jimmy's hand is the key to Abby's room at the Candlewick, and this is interpreted as an invitation to Abby. Despite Henry's objections, Abby departs for the Candlewick alone to confront Charlie. Abby arrives at the Inn and discovers her father waiting in her room. Charlie asks about Jimmy, and Abby tells him he's safe. Abby interrogates him, and Charlie explains that he made a deal with John Wakefield to keep Jimmy alive. Abby reminds Charlie that Wakefield is dead, and Charlie confesses that he shot Wakefield, but never found his body. Charlie tells Abby to leave and Abby refuses, but Charlie snaps at her to stay away as he is a trap. Charlie assures Abby that Wakefield is alive, and he tells her that she is not related to Wakefield.
Abby notices a rope tied to Charlie's wrist, and she begins crying. She then notices a walkie-talkie nearby, and Charlie tells her that it's too late to save him. Abby realizes Charlie traded his life for Jimmy's. Charlie says that Wakefield won't stop killing, but that Abby can finish Wakefield. Charlie tells Abby he loves her before he is pulled backward through the window. Abby soon discovers he had been tied, through an elaborate pulley system, to his own SUV. She finds him outside, hanging from a beam.
- After J.D.'s death, Charlie was the next major suspect for the killer.
- Charlie is the first character to be killed by Wakefield after he is revealed as one of the killers.