Having acquired a reliable fence and ingratiated himself in the upper crust of society. Leslie was ready to start on his true purpose in coming to New York—crime.
In all, Leslie would be an active criminal from 1869 until his death in 1884. During that time, it’s estimated that he played a role in 80 percent of all bank robberies in the United States. Throughout this time, Leslie used his background in architecture to study the blueprints of various banks, uncovering their weak points and using sophisticated tools—some of his own invention—to break into the vaults in a way that didn’t destroy what was inside.
Leslie showed Mandelbaum his skills in June of 1869 with the Ocean National Bank robbery in New York City. In order to pull off the robbery, Leslie wanted to use a device that he’d spent a lot of time perfecting. He called it the “Little Joker”—a small metal wheel with a wire that fit perfectly inside the combination dial of a bank safe. When the safe was then opened, the dial would make notches in the wheel effectively showing the combination of the safe. But there was an obvious catch—you had to get to the vault twice. Once to place the device, and once to remove it.
Leslie became a member of the bank in order to get an idea of the layout and the employees. Mandelbaum secured Leslie a crew—including people like “Jimmy” Hope and “Shag” Draper, whom Leslie would work with again many times in the future. Leslie’s plan was precise, and he even got Mandelbaum to invest in the same safe the bank used so the crew could rehearse multiple times. They rented the empty office space in the basement below the bank. Jonny Irving, who was working as a janitor at the bank, planted the “Little Joker.” Finally, according to Conway in his interview with Slate, the crew broke into the bank on a Saturday (in order to have as much time to complete the job as possible) by drilling through the floor of the bank (or the ceiling of the basement) and ransacking the place. When they were done, the New York Herald reported, they left a mess of tools and goods they couldn’t carry away.
Scattered promiscuously about the floor were United States bonds and currency, gold coin, pieces of … iron, small wedges, railroad bonds, copper coin, augers, chisels, jackscrews, lanterns, fuses, flasks of powder, cigar stumps, ropes, saws, gold certificates and other articles too numerous to mention.