She is first seen in a conference in Los Angeles when she meets the chairman of Nightrise via webcam and discusses electing Charles Baker instead of John Trelawney because Baker is a fascist which suits the Old Ones. The chairman asks another of his men if Trelawney can be assassinated, and the man says it is impossible, saying its obviously a joke. The chairman says that the agent, Simms, ought to consider all options. Simms still disagrees and later Susan Mortlake kills him in New York.
The chairman is a bit disappointed after hearing of Susan's failure at getting both Jamie and Scott Tyler as she only got Scott. So the chairman orders her to make a "redundancy" but considers executing her for her failure. The chairman orders her to kill one of the agents who kidnapped Scott, and she orders her agent Banes to kill his partner Kyle Hovey instead.
Later, Susan goes to meet Scott and convert him to the evil side.
Susan Mortlake spent time with Scott telling him about the creatures she worked for and how Scott would be seeing more of her. Then Banes arrived and began to drug Scott and manipulate him. Susan explained how Scott was always being abused and left behind and how none of it was his fault but rather the fault of his adopters. Then Susan Mortlake dealt some incurable damage to Scott's mind, fully converting him, and blocking out any other telepathic intrusions; even making him build a mental wall to block out Jamie's thoughts.
When Jamie finally met Scott in Nevada, at Trelawney's birthday, Susan Mortlake was with Scott and she was going to order him to get Warren Cornfield, Trelawney's right-hand man, to shoot Trelawney dead, and then to have Cornfield say he'd broken down in stress and it wasn't his fault. Meanwhile, Baker, the other politician, would end up becoming president in Trelawney's place. Susan Mortlake had orchestrated this and she ordered Scott to order Cornfield to shoot, but when Jamie tried to make contact, he was thrown back by the wall. So, Jamie ordered Cornfield not to shoot Trelawney, but Susan Mortlake. He gave the order telepathically and so Cornfield had no choice but to kill Susan Mortlake instead of his boss.
Susan Mortlake was perhaps the most mysterious antagonist of The Power of Five as nothing was known about her apart from her obssession with evil and her sadistic nature. If Jonas Mortlake, her son, could be compared to Barty Crouch Jr. from Harry Potter, then she herself could be compared to Dolores Umbridge from the same series. Susan was described as very unattractive, with a long neck, short gray hair, and loved wearing black. She didn't flatter herself or others, even her own son hardly disguised his contempt for her appearence or personality.
Susan Mortlake was the counterpart of Susan Ashwood from the Nexus corporation, and Susan Mortlake had been given black magic powers by the Old Ones because of her pale skin, her luminous aura, and her cold sadistic nature.