Margaret Gilleo, who turned a local dispute over an antiwar sign on her lawn into a freedom-of-speech showdown at the Supreme Court, died on June 8 at her home in Ladue, Mo. She was 84.

Her husband, Charles J. Guenther Jr., said the cause was pancreatic cancer.

Ms. Gilleo (pronounced GILL-ee-oh) never set out to be a First Amendment crusader. But she refused to allow her rights to be impinged upon, even if neighbors questioned her patriotism or her propriety.

“I consider myself extremely patriotic,” she told The St. Louis Post-Dispatch in 1991. “I love this country. I love the flag, and I would never burn it. But I reserve the right to dissent.”

It all began in December 1990, when the United States was on the verge of the Persian Gulf war. Ms. Gilleo expressed her opposition to the conflict with a 3-by-2-foot sign that said: “Say no to war in the Persian Gulf. Call Congress now.”

Ms. Gilleo lived in Ladue, a wealthy, leafy suburb west of St. Louis that had a longstanding reputation as an exclusive community, filled with residents who demanded a certain level of aesthetic beauty.

Her sign was stolen, and a replacement was pulled up and tossed in the yard. When she complained to the police, they told her that she was at fault: She had violated a local ordinance that forbade virtually all yard signs, which it defined as a form of visual pollution.

The City Council could make exceptions to the rule. But when she asked, its members unanimously voted against her. So Ms. Gilleo took Ladue to court, with the help of the American Civil Liberties Union.

Ladue’s mayor, Edith J. Spink, said that the rationale for removing the sign was strictly aesthetic, not a reaction to Ms. Gilleo’s antiwar stance. Yard signs “would give a cluttered look — visual blight,” she said in testimony at the U.S. District Court for the Eastern District of Missouri, The Post-Dispatch reported. But Ms. Gilleo said she had put up a yard sign earlier supporting an environmental initiative, and that it had not caused a stir.

The case took a circuitous path to the Supreme Court. A district court injunction stopped the ordinance, which a judge called “unconstitutional on its face.”

But the City Council drafted a new ordinance, which allowed a broader range of signs. Ms. Gilleo promptly challenged the rule with a sign in her window that said “For peace in the Gulf,” and the district court struck down that ordinance as well. Ladue appealed, and the U.S. Court of Appeals for the Eighth Circuit affirmed the lower court’s decision.

Even as legal fees mounted, Ladue brought the case to the Supreme Court, which agreed to hear it in 1993.

“I’m really shocked” that the case went on so long, Ms. Gilleo told The Post-Dispatch at the time. She added: “A lot of very powerful people live here; they may feel threatened. You don’t challenge the government on something like that, at least not in Ladue.”

The Supreme Court heard arguments on Feb. 23, 1994. Justice Antonin Scalia noted that a provision of Ladue’s ordinance that allowed rectangular flags but banned triangular pennants was “a pretty stupid judgment.”

“Tell me why — why triangles are worse than rectangles,” he said, drawing laughter from observers.

That June, the court issued a unanimous decision in support of Ms. Gilleo. Justice John Paul Stevens noted in his opinion that “residential signs are an unusually cheap and convenient form of communication,” and that restricting them in the arbitrary manner outlined in Ladue’s law violated the First Amendment.

“A special respect for individual liberty in the home has long been part of our culture and our law,” Justice Stevens said. He added, “That principle has special resonance when the government seeks to constrain a person’s ability to speak there.”

Ms. Gilleo said she was delighted, but not surprised.

“I expected to win all along,” she told ABC News after the ruling.

Margaret Mary Odile Pfeffer was born in St. Louis on Jan. 23, 1939. Her father, Francis Joseph Pfeffer, ran a metal company, and her mother, Ruth (Gander) Pfeffer, was an equestrian and horse breeder who helped establish a charity horse show in St. Louis.

Margaret grew up mainly in Creve Coeur, a city on the outskirts of St. Louis, and graduated from Villa Duchesne, a nearby private school, in 1956. She earned a degree in music from Maryville College (now Maryville University) in 1960 and married Peter Muckerman, a management consultant, the same year.

She taught music at Villa Duchesne and had three children before she and Mr. Muckerman divorced in the mid-1960s. About a decade later, she married Alten Gilleo, a physicist, and moved to New Vernon, N.J.

After Mr. Gilleo died in 1980, she moved to Philadelphia, where she worked in the development office at the Academy of Vocal Arts. She later moved to Manhattan and, after earning a master’s degree in business administration from Columbia University in 1987, worked for a bank. She moved back to St. Louis in 1989.

Ms. Gilleo was working at the nonprofit St. Louis Economic Conversion Project, which sought to teach people about the hazards of the military-industrial complex and to divert U.S. military spending into other areas, when she filed the lawsuit. In 1994, soon after her Supreme Court case, she ran for Congress, but she lost a Democratic primary to Patrick Kelly, a patent lawyer. (He lost the general election to James Talent, the Republican incumbent.)

In addition to her husband, Ms. Gilleo is survived by two daughters, Lyle Seddon and Elizabeth Muckerman; a son, Lawrence; two stepsons, John Guenther and Louis Smith; a stepdaughter, Sarah Guenther; a brother, Joseph Pfeffer; eight grandchildren; and a great-granddaughter.

After Ms. Gilleo’s Supreme Court victory, she completed a master’s degree in theology at the Aquinas Institute of Theology in St. Louis in 1998. In the mid-2000s, she began teaching comparative religion and philosophy at Fontbonne University in St. Louis. She retired in 2014.

One unexpected outcome of Ms. Gilleo’s lawsuit was her marriage to Mr. Guenther. They met when he wrote her a supportive letter in 1993, after the Supreme Court decided to take her case.

As a result of the Supreme Court decision, Ladue had to pay a total of more than $335,000 to cover Ms. Gilleo’s legal fees as well as its own, The Post-Dispatch reported in 1995.

“Somebody told her it was a very expensive personal ad,” Mr. Guenther said.

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