FORTUNE — Thomas Cox, 70, is not a professional homebuilder. Nonetheless, Cox found himself working in construction in Maine for about six years during the 2000s. Those were some of the more rewarding years of his career, he says.

Cox’s inspiration to build homes hit him suddenly after spending decades watching them get destroyed. A former managing partner at a prominent Maine law firm, Cox worked for 25 years helping banks engineer foreclosures. When the savings and loans crisis hit the U.S. in the late 1980s, Cox worked with the Federal Deposit Insurance Corporation collecting faulty loans from members of his small Maine community.

“It was really difficult, dark work, but it needed to be done,” says Cox. “Taking people’s homes is extremely unpleasant. Shutting down their businesses when they are fighting like crazy to keep them open is worse.”

Cox made $300,000 a year during the peak of his career, but said no amount of money could shake the significant toll the work was having on his mental health. Suffering from bipolar disorder and depression, the Pennsylvania native decided to leave the practice entirely in 1998 to pursue treatment options.

A few years later, after battling through a divorce and a slew of treatment options, a former client approached him about joining his carpentry business. The timing was perfect: Cox wanted to get back to a daily work routine, and he always enjoyed projects that produced tangible results. “As a lawyer, when you are done with a day of work all you have is a pile of papers to look at,” explains Cox. “When I built something with my own hands, it took a lot of the pressure off.”

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Medical professionals urged Cox not to go back into the legal profession. Stress is a major trigger of depression and he didn’t need a doctor to tell him that a majority of his anxiety was work-related. Yet when the foreclosure crisis hit the country around 2008, Cox found himself wanting to get involved. Around the same time, a local nonprofit was launching an organization to provide legal help for low-income homeowners facing foreclosures: Main Attorneys Saving Homes.

Once again, Cox felt a call to action to build homes — this time in a different way.

That year, he began taking on cases for MASH on a pro bono basis. The organization considered Cox to be an in-house expert. After all, he wrote the book on foreclosures in Maine — literally. In 1989, he authored a text that outlined the most effective and legal foreclosure methods. So when he started digging into client cases it wasn’t difficult for him to spot instance after instance of bank malpractice when seizing homes and businesses.

“I was stunned by the abuses I was seeing,” he says. “I saw extraordinary sloppiness and outright deceivability of the legal system.”

One 2009 case in particular set off the alarm bells in Cox’s strict law-abiding head. A large mortgage servicer firm at the time — GMAC Mortgage — wanted to take the home of his client, Nicole Bradbury. Cox sensed something wasn’t right about the GMAC employee who signed the foreclosure affidavit against Bradbury. The employee swore under oath that he had personal knowledge of information that Cox knew was impossible for him to know about. After more than year of working on the case, the employee in question admitted to signing thousands of foreclosure affidavits for GMAC without even looking at the papers associated with the case.

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The discovery allowed Bradbury to keep her home, but also exposed the prevalent malpractice later known as “robo-signing” foreclosure contracts. GMAC was forced to suspend all its foreclosure activity. That same year, the nonprofit Encore awarded Cox $100,000 for using his professional expertise to serve the common good.

“It is hard to describe the feeling of saving someone’s house,” Cox says. “Knowing that if I had not contributed, what all those people would have lost is really powerful.”

Today, Cox is working to instill the same passion and staunch abidance for the law that brought him back into the profession for the second act of his career. Traveling around the country on a monthly basis, Cox has spoken to thousands of lawyers about best practices in foreclosure law. He is also still taking on cases with MASH where he has assisted hundreds of families get their homes back.

The workload is approaching the demanding hours that Cox, who is set to get remarried this Fall, remembers from his corporate career with the big banks. But the stress is nowhere to be found.

“My future wife is kicking me,” he says. “I am working way more than 40 hours a week, but I am loving it. I am thriving on it.”


Advice for retirees considering a move from corporate to nonprofit

“Find something that you can really have a passion for and that feeds your soul. My profession that I loved, private practice, has lots of challenges, but it is a noble passion. Now I found a way to work within it to make it better and that feeds my soul.”

What he wishes he knew before the switch

“Working pro bono is limited. Also on the flip side for me personally, I have so much energy for what I am doing it became a problem to pace myself.”

 Biggest challenge 

“Staying organized. One of the things that I just starting doing reasonably well is starting to collect and organize everything that I learned. I had to build a structure as I went along. I don’t know if I could have done it without staying organized.”

 Biggest reward

“When I was in private practice, I had law partners that I was seeing every day. I maintain relationships with some of them today, we took great pride in representing our clients well, but the relationships were essentially all about money. Now working with Main Attorneys Saving Homes, everyone has a mission of doing good work and helping people in need and money is not in it at all. It is a shared sense of doing good work and that is very powerful.”

Next is a series of articles that looks at executives’ efforts to use their talents and skills to enrich or support the lives of others and upon retirement from professional life.