Harry Mailer was an inmate of Stangmoor Prison.
Mailer was scheduled to be subjected to the Keller Machine after George Patrick Barnham. He staged an escape attempt with Lenny Vosper but he and Vosper were recaptured. Shortly afterwards, he was visited by the Master, who helped him take control of the prison. The Master used him and the other inmates to hijack the Thunderbolt missile.
When UNIT attacked the prison, he took the Third Doctor and Jo Grant out of their cells, planning to use them as hostages. He warned them he could get out with either one of them, and if either of them gave him any trouble, he'd shoot them.
While walking down a flight of steps, Jo managed to push Mailer over — but Mailer grabbed hold of her, using her as a shield. He raised his gun at the Doctor and prepared to shoot, but was fatally shot by the Brigadier. (TV: The Mind of Evil)