Margaret Lang lived on a smallholding in Cartympen, with her husband and her daughter. She was a middle-aged woman, and performed the role of a midwife whenever her services were required in the local area. She is sometimes referred to as 'pinched Maggy', no doubt due to the nip that she received from the devil after she had renounced her baptism. Margaret was named by Elizabeth Anderson, John Lindsay, and Thomas Lindsay as being one of the crew of witches that was threatening the local area. She was also indicted by other witnesses during the trial. She was presumably attuned to the local gossip, given that she became aware that she had been accused before any official had approached her.

Instead of waiting for a representation of the Commission to charge her, Margaret immediately visited Bargarran house with the intention of confronting Christian Shaw. When she entered Christian's bedchamber, she asked the girl if she was truly one of her tormentors. The girl replied that she was not. Margaret retired to the hallway, upon which it is said that Christian Shaw experienced a violent fit, before trying to say to witnesses that Margaret had indeed been one of her chief tormentors. Still sitting in the hallway, Margaret undoubtedly heard the commotion. She was said to have professed that she hoped that God would ding, or drive, the devil out of the girl. Margaret then got up to leave.

After she had left the hallway, Christian Shaw recovered enough to directly accuse her once more. Christian claimed that she had heard a spectral voice above her bed just before, which had informed her that a charm had been placed in the room by Margaret when she entered. This charm had bound the young girl's tongue, making her unable to directly accuse Margaret. Upon inspection of the room, a small bundle of hair was found in a place where it was said that Margaret Lang had stood. This was quickly burned in the fire, and Lang was apprehended.

Margaret Lang, like the others who stood trial, was faced not only with the accusation of Christian Shaw, but also other witnesses in the community, as well as those who had been present at Bargarran house when the above events occurred. Her occupation, midwife, is an interesting one, as some people viewed those that performed this duty with suspicion, given that this service was carried out at the cusp of life itself. Some witness statements note that she enquired after a child's health not long before the child died. This echoes the case against Agnes Naismith, who had enquired after Christian Shaw's health not long before the girl's troubles began. At the trial, it is said that Margaret Lang was redoubtable in her defence, and that she was more than an equal for the prosecutor. Reports vary as to her final words before she was executed. Some stories allege that she went to the scaffold admitting to numerous meetings with the devil. Others insist that she prayed fervently for those present at the Gallow Green, those who lived in Bargarran House, and for her own soul, but did not admit to the charges against her.