Roger Dean, who worked at the Quakers' Hill Nursing Home in Sydney as a nurse, pleaded guilty to 11 counts of murder, the BBC said.
Dean, 37, also pleaded guilty to eight counts of causing grievous bodily harm to the residents of the home.
Dean started the blaze in November 2011 at separate ends of the home to distract officers and hospital managers from his theft of 200 prescription pills, Sky News reported.
He was initially hailed as a hero after talking to media outside the home and telling reporters he did whatever he could to get residents out of the blaze.
Dean allegedly told police after the fire Satan had told him to start the blaze.
Judge Megan Latham said Dean was addicted to painkillers at the time of the killing, but there was no evidence he was suffering from a mental illness.