He has championed consumer rights, challenging companies that tacked fees on gift cards and urging judges to shut down lenders that burden borrowers with heavy finance charges.
He has also defended the chairman of a power cooperative against a lawsuit filed by customers and helped fend off a lawsuit against a former federal agent convicted of leaking sensitive data.
The Republican Governors Association has launched an online attack ad depicting the Democrat as a greedy trial lawyer and claiming "there isn't a doctor Roy won't sue." The campaign of his Republican opponent, Nathan Deal, has even labeled Barnes "A Roy Named Sue."
Barnes spokesman Emil Runge says the former governor "believes that being a lawyer is an honorable profession and that lawyers play an important role in protecting our constitutional rights and preserving the freedoms that make our country great."
Sometimes Barnes has helped supporters, such as Superior Court Judge Brooks Blitch, who was accused of using his position to profit.
He also challenged Georgia's voter ID law, which Democrats claimed could disenfranchise Georgia's poor and elderly voters.
In others, Barnes crossed party lines to back former rivals in legal trouble.
Garland Pinholster, once a powerful Republican lawmaker who frequently sparred with Barnes, stepped down from the state Department of Transportation board in June 2008 after he was accused of making sexual comments to two employees. He said he received a surprising call from Barnes, who offered to represent him for free.
"I thought it was very big-league of him, since he wasn't a member of my party," said Pinholster, whose case was settled when the state agreed to pay about $150,000 to the two employees. "I was truly grateful, and I am still grateful."
Barnes began championing consumer-friendly litigation shortly after his election defeat in 2002, working a six-month pro bono stint for the Atlanta Legal Aid Society before starting a law practice with his daughter and son-in-law.
He filed a lawsuit claiming fees and expiration dates on mall gift cards amounted to "stealing by contract" because they lost value after a few months. He challenged the tactics of companies that he said tried to skirt Georgia's ban on "payday" lenders by finding new ways to tack on high fees.
He also championed less popular causes. Among them was his decision to defend Cobb EMC Chief Executive Dwight Brown against angry customers who accused Brown of using the nonprofit cooperative's money to bolster a for-profit affiliate that he ran.
The lawsuit was settled in December 2008.