|“||They [Lucas's victims] were transients and drug users and prostitutes. They were useless to society. I gave them the chance to be part of a cure. To be of use. That's science.||”|
— Mason in "To Hell and Back, part 2"
As a young man, Mason had a promising career as a physician. After graduating from medical school in 2000, he came home to the family farm where he decided he was going to sell it due to the death of his father. Lucas, his autistic brother, became enraged and pushed him from a loft in the barn, paralyzing him from the neck down. Because of his desire for a cure, it began to twist his mind, and he became delusional and a sociopathic sexual sadist. Since that day, he has commanded Lucas to steal medical supplies in order to take care of him and abduct vagrants, junkies, and prostitutes off of the streets, kill them, and extract viable materials from their bodies. These materials would be used in experiments for stem cell therapy, so Mason could find a cure for his paralysis. He would lie in his bed, surrounded by mirrors that allow him to see everything around the house, inside and out. He also had a voice-activated laptop, with which he dictated his odd theories and helped him make phone calls.
To Hell and Back
Mason is first seen when his brother Lucas brings a young girl to his window to get his approval. Shortly after, the BAU arrives and finds him in bed, helpless. He demands that they leave, and that his home is his business. The team looks around to see that his home is filled with medical supplies, both mundane and unusual. A bloody table is also found nearby.
After a lengthy discussion and the discovery of a bin full of nearly one hundred pairs of shoes, Rossi returns to interrogate him. There are many mirrors in the room where Mason is able to sadistically watch his victims being slaughtered and instruct his brother. Rossi begins by removing Mason's mirrors so that he cannot see what is going on. He tells Rossi that Lucas had pushed him from the loft and that he has been taking care of him. He says that Lucas is helpless without him and that he is inhumanly strong and dangerous. He then tells Rossi, "If you see him, don't try to talk him down. Just kill him." Rossi discovers that Mason has been instructing Lucas to abduct victims. He then tells Rossi that there is a photo of Lucas in the other room. Rossi gets the photo and goes outside to relay the info. The local constable is then asked to get a warrant for Mason's laptop.
Hotch calls Garcia and asks her to fly in to hack the computer in person, as he does not want to share any info on the phone. After hacking into Mason's computer, Garcia finds that the kidnappings were performed in order to find suitable "donors" from which to extract stem cells. Mason wants to use the stem cells to cure his paralysis. Meanwhile, William Hightower, the brother of one of Lucas' victims, is listening outside and hears that, due to Mason's disability giving him a good defense, he would most likely be exonerated of any direct involvement in the murders. As Garcia and JJ are getting more info from the laptop, William walks in with an FBI-issued shotgun that had been discarded earlier, coldly shoots and kills Mason as the latter smiles in relief, and is arrested.
No official profile was made of Mason, since the BAU focused more on his brother, Lucas. However, their original profile of Lucas was partially accurate to Mason, as he was a sexually sadistic male with a medical background.
- Lucas was using houses along the old Underground Railroad in order to get his victims across the U.S.-Canada border. These houses and their secret tunnels were built and occupied by the Masons during the Civil War and after. The name Mason Turner may have been inspired by these facts.
- Mason seems to be based on Mason Verger, the main antagonist of the novel Hannibal. Along with having the same first name, both characters were also quadriplegics and sexual sadists who resided in pig farms, were crippled by serial killers (although Lucas wasn't a serial killer at the time), and were verbally and emotionally abusive towards their siblings and caretakers.
- Mason is implied to have U.S. citizenship, as he has a Department of Motor Vehicles file detailing a Social Security number (Social Security is exclusive to the U.S. only), though this was never mentioned in the episodes he appeared in.