The Journal said a Walmart spokesman declined to give a reason Mars was leaving and the executive, who was the company's general counsel from 2002 to 2009, could not be reached for comment.
Mars has been with Walmart for more than 10 years. The Journal said he was involved in the investigation concerning allegations that Walmart executives bribed officials in Mexico to facilitate development of stores in violation of the Foreign Corrupt Practices Act.
In 2005, Mars wrote an email to Walmart's Chief Executive Officer Michael Duke about the allegations.
"The attached memorandum summarizes an interview conducted with a former WalMex in-house lawyer. The lawyer asserts in some detail the alleged corruption by various WalMex associates, including senior people," the email said.
"You'll want to read this. I'm available to discuss next steps. PS: Welcome to Wal-Mart International," the email said.
Walmart has said in a regulatory filing that it spent $157 million in 2012 on matters related to the investigations, which are being conducted by the Justice Department and the Securities and Exchange Commission. The retailer contends it is cooperating with the investigations.