Diamond Jack was a showman and a magician.
In an extensive account given by Mike Yates to UNIT, he said that people who watched his show died from being frozen. He was revealed to be responsible for a sudden freeze at Hyde Park during the 1970s which killed many of those who attended.
He and Jo Grant tracked down Diamond Jack to a street show during which he made Grant disappear. Upon confronting him, Yates discovered he knew of Jo Grant, the Third Doctor and his TARDIS. Jack disappeared suddenly but not before leaving Yates with a Jack of Diamonds card with his home address on it.
Tracking Jack to his home in South East London somewhere near Broccoli Street, Yates found Jo and rescued her. However, before both could leave, they were confronted by Jack. Both were saved from harm by Jo confusing Jack by asking him of his past which was actually a memory of Jo's related to her father . The timely intervention of the Third Doctor and Brigadier Alistair Gordon Lethbridge-Stewart during this moment of confusion further aided them from harm.
During this time, it was revealed that Jack had no memories of his own. He instead relied on stealing the memories others such Jo's. He was connected to a machine that he and Lethbridge-Stewart found in a prison starship underneath Highgate Cemetery. It saved the memories of an alien:Jack's true self.
Jack was exiled to Earth and, as punishment for the numerous number of violence and misfortune he had created, had his own memories wiped out as punishment. As part of his rehabilitation, he was given a new body as a street magician. However, despite their efforts to eliminate them, Jack still managed to retain his alien powers which allowed him to distort space and matter plus allowing him to sap energy to fuel those abilities.
The Doctor, though successful in helping Jack to recover some of his memories, reverted him back to his previous destructive self. Despite an effort by Yates and Lethbridge-Stewart to stop him, Jack again kidnapped Jo Grant and disappeared. Using the prison starship at Highgate Cemetery, the Third Doctor, with the surprise aid of Jack's sentient though completely memory deprived alien body, used its tracking technology to find him at Tower Bridge.
After being confronted by the Doctor, who failed in his attempt to persuade Jack complete the rehabilitation his people intended him to have, Jack confronted and inadvertently killed his memory deprived alien body who had tracked him down too. This resulted in a fatal psychological bleeding effect in which the memories of his larger alien mind began to flood into his smaller more incompatible human avatar mind.
As a last resort, Jack attempted to forcefully use a still captured Jo Grant's mind as a means to save his own. However, he was shot and killed by Yates before the process could be successfully completed though. (AUDIO: The Magician's Oath)