Mary Winchester (née Campbell) regularly portrayed by Samantha Smith but depicted by Amy Gumenick in the time-travel episodes "In the Beginning" and "The Song Remains the Same", is the wife of John Winchester and mother of Sam and Dean. She was born in 1954 to Samuel and Deanna Campbell, who were hunters, and she was raised into a life of hunting. Dean, sent back in time by the angel Castiel, unknowingly brings Mary to Azazel's attention through his actions. Azazel kills her parents and then-boyfriend John Winchester (a non-hunter unaware of Mary's hunter lifestyle), after which he bargains with Mary for John's life, offering to resurrect John if she allows him to enter her house ten years later. Not knowing Azazel's intentions, she agrees, eventually marrying John and leaving the life of a hunter. Sometime afterward, when she is pregnant with Dean, the adult Dean and Sam visit her again to warn her of Anna Milton, who intends to kill Mary before she could give birth to her sons and thus avert the Apocalypse. She is successfully protected when Michael possesses John to kill Anna, then erases Mary's memories so history remains the same. In 1980, Mary briefly returns to hunting to "tie up some loose ends." This includes traveling to Manitoba, Canada and killing a werewolf that she has some kind of history with. In the process, Mary saves the life of a young boy named Asa Fox, inspiring him to become a legendary hunter in the process.
In the pilot episode, six months after Sam's birth in 1983, Mary was awakened by sounds of him crying in his crib. She discovered Azazel there - later revealed to have been feeding Sam his demonic blood - and confronted him, but was pinned to the ceiling by him and slashed across her abdomen, eventually bursting into flames. According to "The Kids Are Alright", Azazel then killed all of her remaining friends and acquaintances, presumably so her children would not have any leads as to her death. It is revealed in "Home" that Mary has been acting as a guardian spirit of the Winchesters' old home in Lawrence, Kansas. She fights the poltergeist haunting the house and forces both spirits to leave.
As a thank-you gift for making her and God reconcile in "Alpha and Omega", the Darkness brings back Mary as she appeared at the time of her death. She is informed by Dean about the events that transpired in the 33 years since her death; to her dismay, she learns that John is dead and that her efforts to protect her family from the hunter world are futile.
In season 12, Mary struggles to adapt to the world that has changed greatly from what she once knew and the changes in her sons. Mary's struggle leads her to isolate herself from her children while her desire to create a world free of monsters leads to Mary allying with the British Men of Letters, particularly their top assassin Arthur Ketch. Mary is shown to be a hunter with formidable skills, described by Arthur as the best hunter he has ever seen. Mary herself states that she was very good at hunting before she gave the life up. Mary's association with the British Men of Letters leads to her being brainwashed into a mindless assassin used to kill the American hunters. After being captured by Jody Mills, Mary's brainwashing is broken by Dean with the help of Lady Toni Bevell in time for Mary to kill Arthur Ketch and save Dean's life. Reunited with her children, Mary attends to Kelly Kline as she gives birth to Lucifer's Nephilim son Jack and sacrifices herself to trap Lucifer in the alternate reality known as Apocalypse World, trapping herself with him.
In season 13, Mary is trapped in Apocalypse World while her sons attempt to rescue her. Originally trapped with Lucifer, Mary is captured and tortured by the archangel Michael, the ruler of Apocalypse World. An attempt by her sons to rescue her leads to Lucifer's son Jack being trapped with Mary as well. Working together, Jack and Mary escape Michael's fortress and link up with some of the surviving human population led by Bobby Singer. While reminiscing with Bobby about her deceased counterpart, Mary learns that in Apocalypse World, her counterpart never made the deal with Azazel to bring back John as seen in season 4's "In The Beginning." Mary realizes that as a result, Sam and Dean never existed to stop the Apocalypse and receives comfort from Bobby that her own decision to make the deal was in fact the right one after all. Mary helps defend Bobby's colony against an angel attack led by Zachariah and together with Jack, forms a resistance against Michael as learned by Dean and a resurrected Arthur Ketch when they briefly visit Apocalypse World in "Bring 'em Back Alive".
Mary eventually returns to the original world with the refugees from 'Apocalypse World', forming a tentative relationship with the alternate Bobby as they attempt to fight the threat of the alternate Michael as he attempts to take Dean as his Vessel. Michael is eventually defeated when Jack banishes him, but Jack begins to damage his soul as he taps into his powers after a confrontation with Lucifer, which results in him killing Mary by accident. His attempt to bring Mary back to life only restores her body, and when Castiel goes to Heaven to retrieve her soul, he learns that she has been reunited with John Winchester in Heaven, prompting Castiel to let Mary stay dead so she can be with him.
According to series creator Eric Kripke, her relation to Azazel was supposed to be addressed in the third season, but was pushed back to the fourth season due to the 2007-08 writer's strike.