Jilly Greca was one of the first heavy Mob guys that Pistone met and was able to get close enough to to get real information/street cred. At the time Pistone met him Greca was an associate but he had his own crew already. Jilly's primary business interests were hijacking, burglary, robbery and fencing , which fit in quite well with Pistone's jewel thief cover. Supposedly Pistone never went out on jobs with them, which seems sorta suspicious in hindsight, but came up with enough "stolen" property on his own to allay most (not all) suspicions.

Shortly after Pistone began the association, Greco was formally inducted in the Colombo family. Pistone was able to use his association with Jilly as a reference to meet/interact with Lefty and the Bonannos. Lefty knew Jilly. Pistone thought that the FBI would gain more direct information on the Mafia and larger crimes if he slowly transferred the investigation from Jilly's crew to Lefty/Tony Mirra and the Bonannos.