The attorney general's office said in a letter on Monday that it found Tillerson had used an alias email address under the pseudonym "Wayne Tracker" from at least 2008 through 2015.
Wayne is Tillerson's middle name.
The letter was sent to a New York state judge overseeing Schneiderman's investigation into whether Exxon misled shareholders and the public about climate change.
In a statement on Monday, Exxon spokesman Alan Jeffers said, "The email address, Wayne.Tracker@exxonmobil.com, is part of the company’s email system and was put in place for secure and expedited communications between select senior company officials and the former chairman for a broad range of business-related topics."
Jeffers said the company had provided 2.5 million pages of documents in response to a subpoena from Schneiderman's office and would respond to the claims in the letter in court filings.
A State Department spokeswoman declined to comment on the matter.
The letter, seen by Reuters, said Exxon had not previously disclosed the alias account. It asked the judge to order Exxon to explain whether documents from the "Wayne Tracker" email and 34 additional accounts assigned to other Exxon executives and board members had been preserved.
The letter said that Exxon had produced some 60 documents bearing the "Wayne Tracker" email but never said it was used by Tillerson for relevant communications at Exxon.
It asked the court to order Exxon to identify whether any other email accounts were used by Tillerson.
"Exxon's top executives, and in particular, Mr. Tillerson, have made multiple representations that are at the center of OAG's (attorney general's office) investigation of potentially false or misleading statements to investors and the public," the letter said.
The case is People of the State of New York v PricewaterhouseCoopers and Exxon Mobil Corporation, New York State Supreme Court, New York County, No. 451962/2016.