Like with all characters that appear in both Desperation and The Regulators, there are some differences in her character between the two stories since they are set in different fictional universes.
Mary Jackson is a college professor and the wife of Peter Jackson in Desperation. She and her husband are stopped by Collie Entragian outside of Desperation, Nevada for not having a license plate. Collie finds a bag of marijuana in the trunk of the car and takes Mary and Peter prisoner. Peter is killed by Collie, but Mary is one of God's chosen to stop Tak. She survives the battle and leaves the cursed town with the other survivors.
In the Richard Bachman novel The Regulators, Mary and her husband Peter both live on Poplar Street, Wentworth, Ohio. She works at an accountants firm and is secretly having an affair with a married man named Gene Martin.
When she returns home (supposedly from work, but in reality from another meeting with Gene), she finds her home street in a state of chaos due to Tak's creatures having killed Cary Ripton and a dog named Hannibal. As soon as she arrives, her car is rammed from behind by a mysterious yellow van. She is able to get out of the wreck, only to be gunned down by two of Tak's other creatures.