John Mosley is a man trying to locate the Beacon, and willing to do anything to get it. Prior to this, he was wanted for a double homicide in Seattle, WA. Mosley shows up in the home of Colonel Henry Jacobson, with the intent to track down the Beacon. Equipped with an energy gun and a device with which he apparently is able to read Jacobson's mind, he eventually learns the location of the Beacon; Dr. Walter Bishop's lab.
Arriving at the lab Mosley abducts Walter's son Peter Bishop, and by hooking him up to the same device that read Jacobson's thoughts he is able to find out where Walter had hidden the Beacon - the grave of Walter's father, Robert Bishop - even though Peter himself is unaware of this.
Arriving at the cemetery, Mosley has Peter dig up the grave, and is finally able to get his hands on the Beacon. Before he is able to do anything more, Agent Olivia Dunham arrives - Mosley attempts to escape but is shot dead by Dunham. ("The Arrival")
- He has the pattern of 3 green and 1 red on his hat, a recurring motif in the series.
- During a "Lightning Round: Q&A" session with the producers of FRINGE, one question stated: "Will we see more rogue Observers, AKA. John Mosley?" The producers only stated that the audience would soon learn exactly what a rogue observer is.