A opened a store in San Francisco to sell sweaters in the mid-1960s. At the suggestion of a customer, he added 10 inches or so of length to the sweaters and turned them into snug minidresses. When he introduced an acrylic knit dress decorated with rows of peace signs in 1967, he had a hot-selling fashion item, soon dubbed the “peace dress.”

The Gannett News Service called him “one of the most influential and most successful dress designers on the current scene.”

When the peace dress came out, not everyone knew what the symbol meant. Newsday explained to readers that the “upside-down, three-prong branch inside a circle” was a symbol for unilateral disarmament. Some women bought the $20 dress to wear to antiwar protests; others simply liked the pattern.

Mr. Duskin used some of his profits in a successful campaign to block a proposal by Lamar Hunt to redevelop Alcatraz Island as a theme park. After an occupation by Native Americans, Alcatraz became part of the Golden Gate National Recreation Area. He also crusaded against skyscrapers in San Francisco, the Vietnam War and nuclear reactors.

Mr. Duskin sold his garment business in the early 1970s and became a staff member in the U.S. Senate, where he worked on legislation granting tax breaks for alternative energy. He later helped run U.S. Windpower Inc., which developed a wind farm in California’s Altamont Pass. He also formed companies to import hydropower equipment from Russia and process potatoes in Poland.

Mr. Duskin died July 25 at his home in Tomales, Calif. He was 90.

One of Mr. Duskin’s top designers, Marsha Fox, created the peace dress, later included in a 2017 exhibit, “The Summer of Love Experience,” at the de Young Museum in San Francisco.

Mr. Duskin encouraged his employees to be creative, Ms. Fox told the Gannett News Service in 1970. “There are good vibrations here,” she said.

Mr. Duskin’s goal in business, he said, was to fund his political causes, not to get rich. “Any money I make, I don’t keep,” he said. “I don’t buy stuff, and I don’t store money up.”

Although he shared sympathies with hippies, he didn’t embrace all of their visions. “I don’t dig the rural communes,” he told the St. Louis Post-Dispatch in 1970. “They’re not the answer.”

Alvin Lee Duskin was born March 5, 1931, and grew up in San Francisco and Los Angeles. He recalled attending 13 schools as his father moved around in search of work during the Depression. His mother was a seamstress, and his father eventually ran a business making sweaters. Before going into the dress business, Alvin Duskin studied philosophy at Stanford University and what is now San Francisco State University.

He also helped run a tiny, short-lived alternative educational institution, Emerson College, featuring roundtable discussions instead of lectures, in Pacific Grove, Calif. The college was launched in 1960 with a fundraising benefit dubbed an “Orgy of Poetry.”

Mr. Duskin’s move into the garment trade started out as a plan to pay off debts and support himself for a short period while awaiting his next academic job. The dresses were so lucrative that he stayed in the business for more than six years.

“Business is where it’s happening,” he told Playboy magazine in 1970. “You have to work within the system if you really want to change it.”

Mr. Duskin, who wore denim shirts and described himself as a “pacifist-anarchist,” said he paid his garment factory workers well above union scale. “I believe in the union movement,” he told United Press International in 1970, “but I don’t like unions. They betray the interests of the workers.”

He questioned some of the clothing industry’s core beliefs. “I think people buy too much clothing,” Mr. Duskin said. He predicted a new age in which women would share “cooperative wardrobes.”

Profits from the business gave him money for such projects as financing a documentary on Fidel Castro and campaigning against construction of buildings of more than six or seven stories in San Francisco. During that campaign, he carried a sign reading: “Stop Them From Burying Our City Under a Skyline of Tombstones.”

In 1971, he married Sara Urquhart, a native of Scotland who had managed the Paraphernalia fashion boutique on Madison Avenue in New York and supervised designers at Mr. Duskin’s dress company. She survives him along with six children and 12 grandchildren. An earlier marriage ended in divorce.

Mr. Duskin called the peace dress “the world’s first socially significant garment” but hoped to be remembered instead for his political activism. In a 2009 interview with the San Francisco Chronicle, he conceded that some of his early political stands had been simplistic. “There’s a lot we didn’t take into account,” he said.

Zoe Duskin, one of his daughters, said her father relished the challenge of finding practical ways to improve the world and never despaired of finding them. “He was endlessly optimistic,” she said.

Write to James R. Hagerty at bob.hagerty@wsj.com