Malcolm was born in 1711, of an Irish family that had come down in the world, and worked as a laundress and charwoman in the Temple. She appears in some contemporary accounts as Mallcombe.
On the night of February 4, 1733, at chambers in Tanfield Court, Lydia Duncomb, eighty, her companion Elizabeth Harrison, sixty, and her maid Ann Price were found dead. The two older women had been strangled and Ann Price's throat was cut. Money and plate were missing.
Malcolm had worked in the chambers. She was found with bloodstained linen and with money hidden in her hair, and was arrested at once.
She was tried at the Old Bailey later that month and convicted. She maintained throughout that she had taken part in the robbery with three accomplices, two brothers named Alexander and a woman, and had kept watch on the stairs while they did the killing. The accomplices were never produced or identified, and the court did not accept the account.
She was hanged at Fleet Street on March 7, 1733, aged twenty-two. William Hogarth visited her in Newgate two days before and painted her portrait, which is now in the National Galleries of Scotland; the print made from it sold widely. That painting is the reason the case is still known, and it is one of the earliest surviving images of a condemned woman drawn from life.