Petrillo's hiring was first reported Tuesday by Vanity Fair.
A partner at Petrillo Klein & Boxer LLP in New York, Petrillo is expected to take a lead role in negotiations with prosecutors if Cohen ultimately opts to negotiate a deal.
As ABC News reported last week, Cohen's current legal team –- Stephen Ryan and Todd Harrison of McDermott Will & Emery LLP -- will remain on the case until those attorneys complete a privilege review of materials seized during raids on Cohen's properties in New York on April 9. Prosecutors and Cohen's counsel have settled on June 25 for that process to be wrapped up. U.S. District Judge Kimba Wood has not yet endorsed the proposed deadline.
Cohen has not been charged with a crime, but he is under investigation for potential criminal violations surrounding his personal business dealings and his $130,000 payment to adult film star Stephanie Clifford aka Stormy Daniels. She said she had a sexual encounter with Trump in 2006. Trump has denied that.
For the past two months, Cohen's lawyers have been rushing to review more than 3.7 million items seized in searches of Cohen's home, office and hotel room. Under the supervision of a retired federal judge acting as a "special master," the files are being scrutinized by Cohen's attorneys for items potentially covered by attorney-client privilege.
Once that task is complete, Petrillo will assume the role of Cohen's lead counsel in connection with the investigation in New York.
Petrillo led the criminal division of the U.S. Attorney's Office for the Southern District of New York from January 2008 until October 2009, according to a biography on his firm's website. He established the firm in 2010 along with two other former federal prosecutors in New York, Joshua Klein and Nelson Boxer.
Previously, Petrillo was an Assistant U.S. Attorney for the criminal division for seven years in the 1990s, including a stint as the chief prosecutor in the division's narcotics unit, according to the firm's website.
"His experience includes hundreds of client representations covering the broad spectrum of criminal prosecutions," his online biography reads, "from alleged financial industry and other fraud, to, among others, alleged money laundering, alleged violations of banking, FCPA and U.S. embargo and sanctions laws, as well as matters involving alleged breaches of public integrity, alleged violations of pharmaceutical marketing enactments, and alleged transgressions of government program and financial industry laws and regulations."