During his tenure with the U.S. Attorney's Office in Massachusetts, Lelling led the prosecution of a billion-dollar pyramid scheme that defrauded almost 2 million investors.

Lelling said in part at the news conference announcing the indictments, "We did not bring this case in response to the public debate over immigration enforcement.

Former Massachusetts Supreme Judicial Court justice Geraldine S. Hines said of Lelling's decision to prosecute that "Maybe he's trying to get on Trump's radar for whatever reason, and this is certainly the kind of thing that's going to get attention in Washington...it's a way to get yourself noticed.

"[10] In contrast, former senior U.S. Justice Department official Ralph Boyd, Jr., praised the prosecution as "principled" and that "it's rare to find public servants willing to enforce the law regardless of status, privilege, or political considerations.

"[33] In September 2022, the Shelly Joseph case was dismissed by Zachary Cunha,[34] a United States Attorney appointed by President Biden.

As part of the agreement to drop the charges, Judge Joseph admitted to certain conduct, including, during the hearing before the undocumented immigrant was released, saying, "Is ICE gonna get him?," before turning off the courtroom recording for 52 seconds.

Lelling also said, "What the agreed factual statement carefully avoids is requiring Judge Joseph to confirm or deny that she intentionally conspired to help the defendant evade federal authorities.

[39]  Convicted defendants included the Chief Executive Officer of Insys, John Kapoor, as well as several managers and sales staff, who had aggressively marketed a powerful fentanyl-based drug for off-label use.

"Experts saw the trial as sending a message to drug companies that they will be held criminally accountable for their alleged role in fueling the opioid crisis.

[42] Begun by the Department of Justice (DOJ) under then–US President Donald Trump's first Attorney General Jeff Sessions in 2018, with the goal of preventing industrial espionage and other theft of U.S. technology for the benefit of the Chinese government.

In January 2020, Lelling announced the indictment of Prof. Charles M. Lieber, a Caucasion male and head of Harvard University's Chemistry Department, for hiding financial ties to China from the U.S. government.

[53][54] On January 14, 2022, about a year after Lelling's departure from the U.S. Attorney's Office, the Wall Street Journal reported that federal prosecutors had recommended dropping the criminal charges against Professor Chen.

"[57] On January 20, 2022, federal prosecutors filed a motion to drop the charges "in the interests of justice", and the US District Court dismissed the case.