Mulally, the 68-year-old executive credited with reviving Ford's fortunes since taking its helm in 2006, will step down before the end of the year, said the person, who asked not to be identified discussing the company's plans. Fields, 53, was named COO in December 2012 and has been seen as Mulally's successor.
"It's really close," the person said. Bloomberg earlier reported that Ford may make an announcement as early as May 1.
A Ford spokeswoman declined to comment.
"We do take succession planning very seriously," Ford spokeswoman Susan Krusel said. "We do have succession plans in place for all of our key leadership positions, but for competitive reasons we do not discuss them."
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In January, Mulally ended speculation that he was in the running for the top job at software giant Microsoft Corp (MSFT.O) and said he would remain at Ford at least this year. He emphasized during the Detroit auto show that month that he remained deeply engaged in Ford's day-to-day operations as well as setting long-term strategy.
However, sources previously said it was unlikely Mulally would stay through 2014 and was looking for another high-profile job. Ford's board was frustrated by Mulally's courtship of Microsoft, the sources said.
Mulally spurred a cultural change that helped the company take quicker action and make bolder bets, analysts, executives and other industry observers have said.
One such risk is Ford's overhaul of its top-selling F-150 truck, unveiled in January. The truck's body is made almost entirely out of an aluminum alloy, which Ford bets will widen its lead over rivals General Motors Co (GM.N) and Chrysler Group LLC (FIA.MI) in the lucrative segment.
In late 2012, Ford first announced that Mulally would remain at Ford until at least the end of 2014. This was part of a series of changes that included the promotion of Fields to COO.
As COO, Fields runs Ford's weekly "business plan review" and manages day-to-day operations.
An exit by Mulally in 2014 would mark the second high-profile changeover in the CEO position at an American automaker as Mary Barra took over at GM in January. She has since been dealing with the fallout from defective ignition switches linked to at least 13 deaths.