A source said Mills was leaving the airline as he was not getting operational freedom to run the organisation.
The difficult period created by some "legacy issues" are over.
Board has also accepted the resignations of Marans.
Kalanithi Maran and another top company official have been summoned as accused by a Delhi court in two tax evasion cases.
The court had on July 21 issued summons to the firm, Maran and Natrajhen and directed them to appear today after taking cognizance of two I-T complaints filed against them.