Mitchell Davis, 72, a retired ophthalmologist who has donated millions to local charities, at his home in Northwest Washington. (Bill O’Leary/The Washington Post)

The unassuming millionaire lives in an aging brick house crammed with more than 50 years’ worth of stuff, including, somewhere, expressions of thanks from a major hospital, a local college and a handful of small nonprofit groups.

He drives a pair of Buicks. “To me, if it has four wheels and moves, I’m happy,” he said. “Buicks have good repair records.”

You would never guess that Mitchell E. Davis has donated more than $4 million to local institutions in the past few years, including $750,000 this spring to Arlington Thrive, a tiny nonprofit that provides emergency funding for medical services to people who have no other way to pay.

“I wanted to give to a charity where my donation would have a significant impact,” said Davis, 72, a retired ophthalmologist who lives in his parents’ old home on a busy intersection in Northwest Washington. “I only know that when I do good, I feel good. Even a small contribution can make a big difference.”

Davis made his fortune by investing in funds in the Standard & Poor’s 500-stock index, after trying and failing to pick stock winners himself.

Mitchell Davis at his home in Northwest Washington. (Bill O’Leary/The Washington Post)

He started with a small inheritance from his mother and father, both Russian immigrants. (His dad arrived in this country as a 10-year-old orphan, put himself through medical school and became a general practitioner.)

Davis decides whom to give to by keeping an eye out for worthy causes, and he says he would never donate to an institution that approached him first.

He read about Arlington Thrive in Washingtonian magazine, asked questions, and checked out its financial reports and reputation in the Catalogue for Philanthropy before starting to give the organization smaller donations — about $35,000 a year — in 2006. He has never visited the facility nor met its executive director.

“I think I’ve maybe been to Virginia once, and I’m not sure I’d go again — my sense of direction isn’t too good and I’d probably get lost,” Davis joked in an interview. “I might get there, but I’d never get back.”

Davis said he agreed to speak publicly about his philanthropy to encourage others to invest wisely, research thoroughly and support those doing good work that will make a difference in peoples’ lives.

But he would much rather discuss the four miles he walks every morning or his twice-weekly workouts with a personal trainer. He declares himself in “perfect health,” despite his rail-thin frame and stooped posture.

A native Washingtonian and a proud graduate of the District’s public schools — Shepherd Elementary, Paul Junior High, Coolidge High — Davis received his bachelor’s degree from George Washington University and his medical degree from the University of Maryland at Baltimore.

He interned and was a medical resident at what is now MedStar Washington Hospital Center in the 1970s, before establishing his medical practice in Rockville. He never married, and he has no children.

At the hospital, he remembers seeing people in the waiting room, “people who took a bus to the hospital and who were dressed in such a way that you knew they lived in poverty.”

“I wished there was something I could do to help them, but at the time, all I had was enough to pay for food and shelter,” Davis said. “I put it in the back of my mind until I accumulated enough.”

Eventually, he decided he was ready. His first gift to the hospital was $1 million to hire a cancer “navigator,” whose job it is to help oncology patients maneuver through the maze of physicians, laboratories and appointments.

A year later, he donated another $1 million to cover prescriptions, procedures, transportation, rehabilitation services and basic equipment such as wheelchairs and canes for those who cannot afford them.

A third million was for patient needs and equipment at the hospital’s affiliate and neighbor institution, the National Rehabilitation Network.

All three gifts are designated as “last resort” funds, to be used only when there is no other source of payment.

Emily Riffle, the hospitals’ vice president of philanthropy, called Davis “an extraordinary philanthropist and such a kind man.”

In the interview, Davis casually mentioned that he also gave some money to Medical Missionaries in Manassas after reading an articlein The Washington Post about its post-Hurricane Katrina work.

The group’s founder, Gilbert Irwin, said that “over the years, [Davis’s] contributions have approached a half-million dollars.”

Since 2009, Davis has been giving to Montgomery College. He said he funded three named scholarships there because he figured hard-working students at the school need the help. One of the scholarships, established last year with a $50,000 endowment, is for students with disabilities.

Part of the fun of giving money away, Davis said, is finding worthy recipients.

“I would never give to anybody who solicited me,” he said.

The $750,000 donation to Arlington Thrive was the largest one-time gift in the organization’s 42-year existence.

Executive Director Andrew Schneider called Davis’s endowment “Sam Walton money” that will have an “unbelievable impact” on the charity, which has only two full-time employees and one part-time worker.

The interest will pay for medical and dental bills for those in need, as well as prescriptions and office visits, covering about a third of the organization’s overall medical and dental spending.

Davis said he has kept enough of his fortune to be generous to relatives, live comfortably and make additional, albeit smaller, donations in the future.

“I felt this money should be doing some good instead of going to Uncle Sam,” he said. “This money could be helping so many people.”