S. David Freeman, an engineer, lawyer and Stetson-sporting “green cowboy” who championed renewable energy, advised three presidents and led some of the nation’s largest public utilities, died May 12 at a hospital in Reston, Va. He was 94.

The cause was a heart attack, said his son Stan Freeman.

Mr. Freeman was an outspoken Tennessee native known for his cream-colored cowboy hat and singular obsession with energy conservation. Working in the 1970s as a White House adviser and Ford Foundation researcher, he emerged as “an energy prophet,” as the New York Times once put it, by arguing that utilities should focus on energy efficiency more than energy production — a minority view that gained growing acceptance over the next two decades.

Long before climate change became a global focus, Mr. Freeman was also a crusader for clean energy, promoting wind turbines, solar panels and the development of electric cars and transportation systems. “The planet,” he declared in 1992, “cannot afford the continued or increased production of coal or oil.”

Mr. Freeman faced enduring criticism for his opposition to nuclear power, which he deemed unsafe, unnecessary and uneconomical, and for a management style described as brash and abrasive. But by 2001, when he was enlisted to negotiate California’s long-term electricity contracts and help the state out of an energy crisis, he was arguably the country’s preeminent leader, and champion, of public utilities.

“It’s time the words ‘public power’ are pronounced again in public,” he told the Times. “It was public power that turned the lights on in rural America. Not too many people are alive today who know that. We’ve had electricity in the whole country for 50 years, but not much more than that.”

As a boy in Chattanooga, Tenn., Mr. Freeman had watched President Franklin D. Roosevelt dedicate the Chickamauga Dam as part of the newly formed Tennessee Valley Authority (TVA), which brought cheap electricity to much of Appalachia. He later launched his career as an engineer and then lawyer with the authority, before following his boss to Washington in 1961 to join the Federal Power Commission.

Mr. Freeman worked in the Johnson, Nixon and Carter White Houses, helping to shape the federal government’s energy policies at a time when the Environmental Protection Agency and Department of Energy were being created.

He also advised the Senate Commerce Committee on fuel-efficiency standards and oversaw the Ford Foundation report “A Time to Choose” (1974), which advanced many of his views on energy efficiency. The report impressed politicians including President Jimmy Carter, who appointed Mr. Freeman to the board of the TVA, the country’s largest public utility, and named him chairman in 1978.

At the time, the authority was working on a host of nuclear power-plant projects and had built a set of coal plants that were reportedly the nation’s worst source of air pollution. Mr. Freeman declared that he wanted to “hook TVA customers to the sun” and “make the valley the Detroit of electric vehicles,” and over the next few years he stopped the development of eight nuclear reactors and installed $1 billion worth of pollution controls on the TVA’s coal plants.

The National Wildlife Federation named him conservationist of the year, although rate increases under his leadership angered many of the authority’s roughly 7 million customers. President Ronald Reagan appointed a new chairman in 1981, and after Mr. Freeman’s term on the board expired in 1984 he spent much of the next two decades in charge of public utilities across the country.

Mr. Freeman led the Austin-based Lower Colorado River Authority, where he quashed plans to build a lignite mine; the Sacramento Municipal Utility District, where he was credited with steadying the utility’s finances after voters elected to close a nuclear power plant; and the New York Power Authority, the largest public power agency outside the federal government.

In 1997 he took over the Los Angeles Department of Water and Power, where he was credited with cutting the agency’s $8 billion debt by half without raising rates. When an energy crisis rattled the state four years later, he was criticized by Republican politicians who said he had gouged state ratepayers when his Los Angeles department, which had ample electricity, profited from selling power to other utilities.

“I’ve been called a lot of bad things,” Mr. Freeman told E&E News, an energy and environment publication, in 2018. “I was called a socialist in the Nixon administration for pushing energy efficiency. I don’t feel like I’m making my case if I don’t have somebody pissed off at me.”

The older of two children, Simon David Freeman was born in Chattanooga on Jan. 14, 1926. His father, an umbrella repairman, was a Jewish immigrant from Lithuania who arrived at Ellis Island, consulted an atlas and moved to Chattanooga because he “saw that it rained a lot there,” Stan Freeman said.

Dave Freeman, as he was known, served in the Merchant Marine during World War II and studied civil engineering at Georgia Tech, then known as the Georgia School of Technology. He received a bachelor’s degree in 1948 and graduated from law school at the University of Tennessee in 1956, after an initial stint working on power plants for the TVA.

His marriages to Marianne Cohn, Anne Crawford and Suzanne Kennedy ended in divorce. Survivors include three children from his first marriage, Anita Hopkins of Reston, Stan Freeman of Bethesda, Md., and Roger Freeman of Denver; a brother; nine grandchildren; and a great-grandson.

In his 80s, Mr. Freeman led an effort to clean up air pollution at the ports of Los Angeles and Long Beach, Calif., and served as deputy mayor for energy and the environment in Los Angeles. He was still making headlines until shortly before his death, criticizing the TVA for not doing more to take advantage of wind and solar power and calling on fellow environmentalists to take action.

“What concerns me is the absolute failure of the Democratic Party or environmental movement to tell my grandchildren what we’re for,” he told E&E News. “What is the green agenda today? To bellyache about [President] Trump? It’s not good enough. Mother Nature doesn’t give a damn to what you say. It depends on what you do.”