Sam and Dean encountered Sheriff Jody Mills for the first time during a visit to Bobby Singer's hometown, Sioux Falls. The brothers had intended to use Bobby as their superior so Jody wouldn't interfere with their investigation while they posed as FBI agents. Jody recognized Bobby's voice and immediately told the brothers off.
When townspeople started becoming zombies, Sam and Dean helped Jody deal with the problem and since then, Jody has requested the brothers help on cases with a supernatural vibe to it. Jody has also become a hunter and one of the few allies of Sam and Dean who is still alive.
In Dead Men Don't Wear Plaid, Sam and Dean travel to Sioux Falls to investigate a murder. They soon discover that dead people are somehow coming back to life and seeking revenge on those who had wronged them. During their investigation, Jody asks to know why they are here and the brothers tell her to speak to their superior.
Unfortunately for Sam and Dean, Jody knows the man pretending to be their superior—Bobby Singer. As Sheriff, Jody tells the brothers off and instructs them to leave the case alone. Sam and Dean soon find out that Jody is unwilling to deal with the mysterious resurrections because her son, Owen Mills, was among the people brought back from the dead.
When the resurrected start turning into zombies, Sam and Dean soon rush to Jody's house. They find Jody's zombified son eating from the remains of his own father, Sean Mills. Sam drags Jody from her house and kills her son for her. Afterwards, Jody helps the brothers kill the remaining zombies.
In Slash Fiction, Jody comes to visit Bobby so she can repay him for saving her life earlier. Jody cooks and cleans for him, the latter which results in the cleaning product to drop onto a captured Leviathan Chet, whom Bobby has hidden in the basement.
In Sacrifice, Jody goes on a date with a man named Roderick, with whom she shares a loss of loved ones. Jody becomes emotional over the memory of her son and husband and tries to calm down, only to start choking on her own blood.
Sam and Dean arrive at their location and reveal that the man Jody is dating is in fact a Crossroad Demon named Crowley. Crowley had targeted Jody among other allies of Sam and Dean in order to force them not to seal the Gates of Hell. As Jody lies dying, Crowley orders the brothers to say "I surrender" and at least one of them does, thus sparing Jody's life.
Jody is the only one of Sam and Dean's targeted allies to have survived Crowley's wrath.
In Alex Annie Alexis Ann, Jody encounters a runaway girl known as "Alex" who is being hunted by vampires. Jody calls in Sam and Dean to help deal with the issue. With the brothers' help, Jody learns that "Alex" is in fact Annie Jones who was kidnapped eight years ago.
While Sam and Dean decide to track down the vampire nest, Jody does her best to protect Annie by keeping her at a family cabin. Annie refuses help however, and goes so far as to coldly talk about Jody's dead family. Despite Jody's best efforts, the vampires manage to capture Annie and take her back to their nest.
Sam and Dean arrive at the cabin the following morning to tell Jody that Annie had been used as bait to lure in unsuspecting humans for her adopted family to feed on. Jody decides to join Sam and Dean on their second attempt to take down the nest. At the location, Jody discovers the leader of the nest, Celia, who wants Annie to feed on Jody to complete her transformation into a vampire.
Because of Jody's kindness, Annie chooses to betray Celia and allow Jody to decapitate the vampire. Meanwhile, Sam and Dean take down the rest of the vampire nest. Sam and Dean use Celia's blood to cure Annie of her incomplete transformation. Jody then takes Annie back to the cabin and admits to the brothers that she wishes to keep Annie in order to fill the void caused by the loss of her own family.
Jody is given the job, as she reunites with Dean who joins her on the hunt which is determined to be a wraith. Missouri has a vision that her estranged son James Turner and granddaughter Patience, who is also a psychic, are in danger. After Dean and Jody leave to rescue them, the wraith kills Missouri. Dean and Jody warn the family but the wraith manages to get to Patience. Dean and Jody track Patience when James reveals a way to find her. Dean eventually manages to kill the wraith and stop the visions coming true. Jody also told a sulking Dean he still had his moves but he is still bitter to accept her compliment and gives a sarcastic remark.
She is also shocked by his agreeing with Patience's father telling her to ignore her powers, and live a normal life. Jody took the girl aside and told her to follow her heart, and gives her a card and tells her to call her if she ever needs to talk.
In The Bad Place, Jody contacts the Winchesters to inform them of the murder of Derek Swan which she has learned from a contact in the Bismarck, North Dakota PD. Though the killing seems like an angel smiting, Derek's girlfriend described him being visited by someone that matched the description of Jack.
In Wayward Sisters, Jody calls Claire to tell her that Sam and Dean are missing after going on a hunting trip and that she should come home. After hearing from Kaia Nieves that the brothers were sent to another world and the rift was still open, Jody and her family went to find it.
When she spotted it, Jody attempted to pass through it and save her friends but Claire and Kaia went through instead and brought them back at the cost of Kaia.
Sam and Dean thanked for her rescuing them and for the food she gave, though Sam warned her the rift was opened for a while and more Cloaked Figures could still be roaming the town. Jody appreciated the worry but told them she and her girls have it under control and tells them to focus on saving the world.
During The Scar, Sam and Dean contact Jody Mills who is happy to hear Dean has return and they were trying to get a clue as to what she knew about a scar that Dean had in her location. As it turns out, Jody had been investigating three murders she thought were done by a human serial killer but realizes were done by the mysterious person when they describe the weapon. The Winchesters and Jody search the woods near Sioux Falls for the person, finding three decapitated vampire heads, confusing them as Alex had determined that the murder victims didn't react to any of the usual monster tests. They eventually realize that the vampires are some of Michael's enhanced monsters that he sent to kill the assailant and get their spear after they injured him. The three find hooded person's camp where Dean recalls Michael was there with the hooded figure. Dean is soon attacked by the hooded person but in the struggle, their hood falls off, revealing their face as Kaia to the shocked hunters before she escapes. Dean tells a confused Jody, he thinks it might be Kaia from another world just like Castiel and Bobby's counterparts, they decide to go after her for answers. After ambushing and tying her up, they interrogate Kaia, they realize that she is indeed the alternate reality counterpart of their Kaia. After Kaia refuses to explain how she opened up a rift between the worlds or where her spear is but remembers how Dean had once threatened the Main Universe Kaia with a gun to get her to comply. When Kaia refuses to give up the location of her spear, Dean threatens to torture her to get it, leading Kaia to taunt him that Dean is no better than Michael. When Dean remembers that Michael approached Kaia to form an alliance and she attacked him instead. The interrogation is cut short by the arrival of four of Michael's enhanced vampires who demand Kaia and the spear. The hunters prove to be no match for the vampires, but Dean manages to shoot out the leg on Kaia's chair, freeing her in the hopes that she will help them. Instead, Kaia flees out a window but unexpectedly returns with her spear to kill the four vampires and save Dean, Sam, and Jody. Sam points out that Kaia saved them when she didn't have to, but she claims that by killing the werewolves she was only helping herself. Though the Winchesters warn her that if she keeps her spear, she will be a target for Michael's monsters, she departs saying that she has been running from monsters successfully all of her life. In the aftermath, they part ways with Jody on good terms.
- Prior to Mary's resurrection and after the death of Ellen Harvelle, Jody was the closest person Sam and Dean had to a mother. Her short-lived relationship with Bobby Singer (Sam and Dean's official adoptive father) also makes her an adoptive mother of sorts.
- Alongside Donna Hanscum, Jody is one of the most recurring allies of Sam and Dean who are also law enforcers.
- It is unknown if Jody knows about the Winchesters indirectly caused her husband's death by starting the apocalypse.