Ford scheduled a 9 a.m. EDT news conference at its Dearborn, Mich., headquarters to introduce Fields as CEO.
Fields, 53, will become CEO after service in a number of positions at Ford over the last 25 years.
Ford has made it clear for more than a year that Fields would eventually replace Mulally . Now that he's officially behind the wheel as the automaker's next CEO, he faces daunting odds.
Historically, CEOs who have followed extremely successful chief executives have struggled to perform as well as their predecessor. In the auto industry, it has been decades since one of the Big 3 automakers had two CEOs put together back-to-back successful and profitable tenures.
Fields and Ford's future
For Fields, one of his biggest challenges as CEO will be keeping Ford operating as one team, delivering steady growth and profits.
Mulally's "One Ford" business plan gave the automaker a central focus after years of losing billions with slew of brands and models that usually struggled.
Fields won't change the "One Ford" game plan. Nor will he make any radical moves for the foreseeable future.
After all, Ford is in the midst of a profitable year where it's expected to earn between $7 billion and $8 billion.
The real challenge for Fields is what happens two or three years from now when the automaker or auto industry faces the next big crisis.
And make no mistake, there will be a crisis at some point.
Fields has been through more than a few major challenges at Ford, so he has seen how the automaker has handled them well (Mulally keeping the company out of bankruptcy during the 2009 auto industry meltdown) and when the automaker lost its way (the Explorer/Firestone recall in 2000).
How Fields handles that first big test of leadership is the big unknown.
Will Ford keep its deep bench?
Under Mulally, Ford developed a deep bench of talented executives.
Joe Hinrichs, who runs Ford operations in North and South America, and Jim Farley who runs the Lincoln brand, have the desire to eventually run an automaker.
Underneath them is a stable of capable and ambitious executives.
That talent should reassure Fields that Ford has the right people to grow the business in the near term.
But how many of those executives will stay at Ford if other automakers come calling three, four or five years from now?
Next generation takes over
With two of the Big 3 now being run by new CEOs who are considerably younger than their predecessors, the auto industry is transitioning to the next-generation of leaders.
Ford and GM have also returned to the historical norm of tapping industry veterans to become chief executives.
Fields, like Mary Barra at General Motors , has seen his automaker go through the worst of times and emerge as a leaner and more profitable company.
Now he's charged with growing those profits.
-By CNBC's Phil LeBeau.