Pat Callahan never planned to become a banker. But the opportunity to shatter stereotypes in a male-dominated industry changed her mind.

Callahan – who retired as Wells Fargo's chief administrative officer at the end of August and on Thursday night accepted a Lifetime Achievement Award as part of American Banker's Most Powerful Women in Banking and Finance program – imagined as a young graduate student that she would wind up at an industrial company like Ford Motor Company. She'd studied engineering at the Massachusetts Institute of Technology and was pursuing an MBA at MIT's Sloan School of Management when she accepted a summer internship at Chemical Bank in New York City.

"I was on the operations side of the bank, which was heavily populated with women workers while all the managers were men," Callahan recalls. "I was an idealistic 24-year-old and thought that I could make a difference."

Looking back on her 38 years at Wells Fargo, Callahan believes she's done just that. The Lifetime Achievement Award winner for 2015 oversaw the largest bank merger in history, between Wells and Wachovia Corp. in 2008. Unlike several other crisis-era deals, the $15.4 billion acquisition went down so smoothly that one analyst labeled it "one of the better acquisitions of all time" in 2014. But Callahan, 62, takes particular pride in the social impact of her work, citing her influence over the bank's diversity policies and environmental practices as two of her biggest achievements.

During her two stints as head of human resources at the San Francisco bank from 1993 to 1997 and from 1998 through 2005, "a lot changed with respect to diversity—things where I made a difference," Callahan says. "We got domestic partner benefits and broadened our nondiscrimination policies. We started a diversity council and really started to think about the importance of diversity in senior management and recruiting."

Callahan says she's also proud of the environmental goals Wells set during her tenure as chief administrative officer, where her duties included managing corporate social responsibility.

"It's blossomed over the last 10 or 12 years from a little effort to save energy and turn the lights off to much more," Callahan says. She easily rattles off the environmental initiatives underway at Wells Fargo, from purchasing smart-water systems to investing in the alternative energy sector through its wholesale banks. The company committed in 2012 to lower greenhouse gas emissions by 35% over eight years and had reduced emissions by 25% at the end of 2014.

"It's something I feel very good about and hope will continue," Callahan says of the bank's environmental focus.

Callahan is equally committed to green living in her personal life. After buying a house in the Berkshires in Western Massachusetts two and a half years ago, Callahan and her husband installed a geothermal heating system and solar panels so the house could produce most of its own energy. They also purchased a SodaStream seltzer maker that enabled them to give up plastic bottles. And they started growing their own vegetables in "a big organic garden that's not quite a small farm," she says.

Callahan says she plans to spend a lot of her leisure time in that garden while she weighs her next move. She's served on the board of the California nonprofit Marine Mammal Center since 2007 and looks forward to doing more work with nonprofits after taking a few months' break.

While moving between the private industry and the nonprofit world could be jolting for some, Callahan has plenty of practice switching gears. Over the course of her varied career at Wells Fargo, she headed up the bank's risk management and compliance department as well as its wholesale banking, finance and operations divisions.

"I moved around a lot and I believe in it very firmly," she says. "You bring the talents and skills you have to a new place, and try to find a way to contribute what you already know while you learn new things."

As Callahan prepares to leave banking behind, she says the industry has made a lot of progress in restoring the public trust that the financial crisis left in shambles. But she adds that banks still need to improve the way they treat customers—or else risk regulatory run-ins as well as further reputational fallout.

"Customers expect things to be really good, and if they aren't, they'll tell everyone on Twitter," Callahan says. "There's also financial risk, because customers will walk away from you and go to another provider."

Wells Fargo is not immune to criticism over its past actions, according to Callahan. 

"I think Wells is a good company and has a strong culture and has done a lot right, but we've also made mistakes," she says. "We're trying to work through them and make sure the mistakes never happen again and that our values are well understood and well communicated. But it's going to take patience."

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