She did not wait to be contacted by criminals instead, Mandelbaum went to them with offers to finance thieves and burglars and was involved in planning some of the biggest theft's in the city's history. Expanding her operations, she controlled several gangs of blackmailers and confidence men as well as a school to recruit and teach younger criminals on pickpocketing. She was also a top competitor to the Grady Gang. Some lauded her work in "rescuing" children and poor women from other predators, but this recent revisionism of Marm Mandelbaum as saviour is, by any reckoning, stretching it a bit.
Alas, all good things come to an end and in 1884, the New York District Attorney hired the Pinkerton Detective Agency in infiltrate Mandelbaum's organisation. An agent, posing as a prospective thief arranged for several marked rolls of silk stolen from a store where it was discovered in a police raid on her home the following morning. Arrested with her son Julius and clerk Herman Stroude, Mandelbaum was released on a $21,000 bail and fled the United States with an estimated $1 million. She settled in Toronto, (Canada at that time did not have an extradition treaty with the United States) and remained there, reportedly happy and in comfort, until her death in 1894.