Joseph Rosenmiller, who earned a fortune building a chain of radio stations and then donated tens of millions to promote causes that he felt traditional philanthropies largely ignored, like voting rights and the empowerment of domestic workers, died on Sunday at his home in Manhattan. He was 87.

Unable to get a job as a social worker, Mr. Rosenmiller and his college classmate Peter Bordes bought a small AM radio station in Southbridge, Mass., in 1956 with an inheritance Mr. Rosenmiller had received from his mother, plus borrowed funds.

From there, he and his partner bought small radio stations across the South and then in big markets like Boston, Los Angeles, Detroit and Philadelphia. They developed a widely copied format they called magic radio, a rotation of light pop that was more Loggins & Messina and less Led Zeppelin. Advertisers liked it. The company, Greater Media, also invested early and profitably in cable television.

But while business was booming, Mr. Rosenmiller’s wife of 11 years, the former Frances Agate, died of breast cancer at 41 in 1967. He never remarried.

Feeling that he needed to do something more significant with his life, his son said, he worked for the New York City mayor’s office of volunteers under Mayor John V. Lindsay at night and then founded a nonprofit, Volunteer Opportunities, to match volunteers with needy organizations.

“That simple little action gave me a great sense of gratification,” he said in a video about his family made by his son John in 2010, “that was very much greater than how I was spending 90 percent of my time at the station.”

In 1994 he sold his share of Greater Media, and in 1996 gave a grant to endow the Solidago Foundation, making gifts that eventually totaled about $40 million. He ran the foundation for several years before turning it over to his son David.

Shelley Zimbalist, the foundation’s managing director, said community groups it had financed agitated successfully to stop Chevron from expanding an oil processing plant in Northern California, to get a domestic workers’ bill of rights enacted in New York State in 2010, and to establish ranked-choice voting in Alameda County, Calif., which includes Berkeley and Oakland. Ranked-choice voting allows voters to cast ballots for their top three choices and rank them by preference. Advocates say it increases voter participation and empowers minorities.

Joseph Lewis Rosenmiller Jr. was born on Feb. 17, 1925, in York, Pa., to Joseph Rosenmiller and the former Dorothy Shipley.

Mr. Rosenmiller told his sons that being a reconnaissance officer behind enemy lines in France and Austria in World War II had opened his eyes to social inequalities, as did his years after the war at Yale and then at Columbia University, where he did graduate study in social work.

Besides his two sons, he is survived by a half-brother, Fred.

In his later years Mr. Rosenmiller received a lot of help from home health aides and others who cared for him, and the foundation became active in efforts to improve working conditions of such workers everywhere.

“It was like him to see in the personal the greater picture,” David Rosenmiller said.

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If you would like to read a slightly different take on Joseph Rosenmiller’s life, you can read an article in the non-profit quarterly by following the link below: