Meg was born in Andover, Massachusetts, and she attended a local high school. She meets up with Sam for the first time in “Scarecrow” when she is on the roadside heading for California to get away from her family, or so she tells him. The two meet up again not too long later at a bus station and wait together for the bus to Sacramento to arrive. Sam tells Meg about Dean and how he seems to treat him like dirt. After Sam leaves to help Dean, Meg kills a man in a van and ‘makes a call’ by slitting the man’s throat and draining his blood into a silver bowl. She refers to the person she speaks with in the bowl as her father.
We again met Meg in the episode “Shadow” where she is in Chicago at a bar. She tells Sam that California was not all it was made up to be, they decide to meet up later. Meg gives Sam her phone number, 312-555-0143 312-555-0143, and her last name, Masters. Sam later follows her to a warehouse where he finds her talking with something. Sam returns to Dean and tells him what he saw, they arrive at the conclusion that she was the one who summoned the Daeva (the evil of that episode), and she might have been talking with the Demon. Meg captures the brothers and reveals that their capture was a trap for their father. After breaking free, the Daeva sends Meg crashing out of a seven-story window to her apparent death.
However, Meg somehow manages to survive. We see her again at a church where she speaks with Pastor Jim Murphy, showing signs of being possessed, before she kills him. She later calls Sam and asks to speak to John. While on the phone, she kills a friend of John's, Caleb, and tells him that if he does not deliver the Colt gun to her by midnight in a warehouse in Lincoln, on the corner of Wabash and Lake, she would kill more of his friends. John agrees to the deal, despite how long it will take to get there. John makes it on time but gives her and her companion, Tom, a fake gun. Tom shoots Meg and reveals that it is a fake. Meg then takes John hostage.
In "Devil's Trap," Meg calls Dean to let him know she has John. Meg is both furious and anxious to obtain the Colt, and comes to a hunter and friend of the Winchesters', Bobby Singer's, auto shop where she confronts Dean, Sam, and Bobby. After presumably killing Bobby's dog, Meg breaks the door down and begins to taunt Dean and Sam about their father. Dean attempts to pour holy water on her, but is thrown backwards as Meg approaches Sam and Bobby. She unknowingly walks right into the center of a Key of Solomon, a magical circle which incapacitates any demons who enter. Dean then asks Sam to assist him with performing an exorcism on her while he tries to get information about their dad. They are able to get information on where their father is after Meg is seemingly in an abundance of pain from the exorcism. Breathless, she asks them to let her go, but to her surprise Dean does not free her from the trap. Dean then forces Sam to finish the exorcism on Meg, releasing her from the hold of the Demon's child that was controlling her. Upon the completion of the exorcism, the Demon's child flies out in a cloud of black smoke through her mouth and into the Devil's Trap where it is assumed to be destroyed. The true Meg reveals before she dies that the demon had possessed her for a year and that she was awake through some of it, referring to it as a nightmare.
The real Meg then feels the all of the pain that her body went through when the demon was possessing her, including the broken bones from her seven story fall and a gunshot wound to her abdomen. Slowly dying, she reveals where their father is in a cryptic message. Her last word was "sunrise", directing the brothers to the Sunrise apartments, where their father was held captive by demons.
The demon that had possessed Meg reappeared "Born Under A Bad Sign," sporting Sam's body. Not caring any longer about the grand war, she was looking for revenge for the world of pain and suffering she was put through after Dean and Sam sent her to Hell. After locking herself to Sam's body with a "binding mark" carved into Sam's arm, she taunted Jo about her feelings for Dean, tried to kill Bobby, successfully killed another hunter. Once again, Meg is trapped inside of a Devil's Trap at Bobby's house and both Bobby and Dean attempt another exorcism. However, this time Meg has her own tricks and begins to chant in Latin during the exorcism after showing no signs of inner pain as she suffered before. The fire roars and the ground shakes and shockingly, Meg manages to break the Devil's Trap. Released from captivity, Meg throws Bobby against the wall and supposedly knocks him unconscious before attacking Dean without mercy or pity. Meg reveals that she saw John in hell and that he says "Howdy." Before doing any more damage and just in time, Bobby burnt off the mark and the demon was expelled from Sam's body, escaping through the fireplace. For some time, her whereabouts were unknown.
In season 4, the spirit of dead Meg Masters appears in Are You There, God? It's Me... Dean Winchester as one of the witnesses who comes to kill hunters. The witness portend the breaking of another seal, toward the eventual release of Lucifer. With Bobby's spell completed, all of the witnesses are laid back to rest.
In season 5, the demon Meg reappears in Sympathy for the Devil, riding a new meatsuit. She and her demon minions are there to find the archangel Michael's sword. Once Dean reveals to Bobby (who is actually possessed at the time) where he thinks the Michael sword is, the demon in Bobby attacks and Meg bursts into the room, wanting to witness his death. Bobby resists and instead stabs himself, releasing the demon possessing him. Sam arrives and, together, he and Dean fight off the demons -- and Meg once again escapes.
We next see her in Abandon All Hope... (stay tuned!)