Mary K. Gaillard was 16 and still known as Mary Ralph when a boy in her neighborhood asked her what she wanted to do with her life. She told him that she wanted to be a physicist.

“A singularly unfeminine profession,” he replied.

Decades later, that remark would inspire the title of Dr. Gaillard’s memoir, “A Singularly Unfeminine Profession: One Woman’s Journey in Physics” (2015), in which she recounted a career spanning a golden age of particle physics, when the outlines of how nature behaves at subatomic scales were just beginning to emerge.

Dr. Gaillard contributed key insights to what is now known as the Standard Model — scientists’ best theory about the properties and interactions of elementary particles — while overcoming discrimination as one of the few women in her field and inspiring other female physicists to do the same.

Physics was “her life,” her son Bruno said. “She was consumed by it.”

Known to many as Mary K, sans period, Dr. Gaillard, who died on May 23 at 86, was the first woman hired by the physics department at the University of California, Berkeley, and later became a senior scientist at Lawrence Berkeley National Laboratory. But much of her groundbreaking work occurred earlier, during a long stint as an unpaid visiting scientist at the European Organization for Nuclear Research, or CERN, a laboratory on the Franco-Swiss border.

She was “brilliant at doing calculations,” said John Ellis, a physicist at King’s College London, who collaborated with Dr. Gaillard at CERN. “If she calculated something, you could be sure that it was correct.”

Dr. Gaillard made dozens of calculations that predicted the properties and behaviors of particles, some yet to be discovered. Colleagues described her as having a knack for translating theory into experimental methods that physicists could use to prove the existence of such particles.

In the late 1960s, experimental results began to suggest that protons and neutrons were made of even smaller particles, known as quarks. Theoretical calculations hinted that these quarks were tightly bound together by other particles called gluons.

Working with theorists at CERN, Dr. Gaillard deduced that gluons would sometimes appear after a collision of electrons and positrons, the positively charged version of electrons. Physicists in Germany discovered gluons through this process in 1979.

In 1975, Dr. Gaillard and her collaborators determined that the Higgs boson — also known as “the God particle,” as it plays a role in imbuing all other particles with mass — would sometimes break down into two photons. Scientists confirmed the existence of the Higgs boson through this process in 2012, a discovery that earned Peter W. Higgs, of the University of Edinburgh in Scotland, and François Englert, of the Université Libre de Bruxelles in Belgium, a shared Nobel Prize in 2013.

Mary Katharine Ralph was born on April 1, 1939, in New Brunswick, N.J., to Philip Lee Ralph, a history professor, and Marion (Wiedmayer) Ralph, a schoolteacher and counselor. Mary K grew up in Painesville, Ohio, and developed an interest in physics in high school.

In 1960, she graduated from Hollins College (now Hollins University) in Virginia with a bachelor’s degree in physics. Later that year, she enrolled in the graduate physics program at Columbia University. She married a physicist, Jean-Marc Gaillard, in 1961 and followed him to France.

At the time, women were often discouraged from pursuing theoretical physics, she wrote in her book, so she assumed that she would become an experimental physicist. But in France, she struggled to find a laboratory to work in. One scientist chided her for moving to France for marriage rather than physics. Another told her that his lab accepted only those who had been students at two all-male French schools; still another rescinded his offer after learning that she was pregnant.

“But it never occurred to me to give up,” Dr. Gaillard said in an interview with UC Berkeley News in 2015. “It was something I loved.”

Her high exam scores earned her admission to the theory group at the University of Paris at Orsay. In the 1960s, France’s academic system offered two levels of doctoral degrees. She earned her first doctorate in 1964, and began working at France’s National Center for Scientific Research, or CNRS, although she spent most of her time at CERN; she earned her second doctorate in 1968.

At CERN, Dr. Gaillard gained recognition for her calculations, all while juggling her research with frequent work-related travel and raising three children. She established and led a theory group at Annecy Particle Physics Laboratory in France in 1979, and became a director of research at CNRS in 1980. But CERN lab officials never offered her a paid staff position.

“They were getting the benefit of her, and not affording her the status that she should have had,” Michael Chanowitz, a physicist at Lawrence Berkeley National Laboratory, said in an interview. He collaborated with Dr. Gaillard in predicting the mass of the so-called bottom quark.

Visits to the Fermi National Accelerator Laboratory, or Fermilab, in Illinois offered a respite for Dr. Gaillard. In 1973, she began collaborating with Benjamin Lee, the head of Fermilab’s theory group. She and another physicist predicted the mass of a then-hypothetical quark called charm, and how the particle might show up in experiments.

Physicists detected the charm quark in 1974. The discovery won two more scientists a Nobel Prize and convinced the physics community that quarks were real.

“She was on a whirlwind with the work she was doing,” Shirley Ann Jackson, a physicist and former president of Rensselaer Polytechnic Institute in New York, who met Dr. Gaillard at Fermilab, said in an interview. “At that point, I didn’t realize how much discrimination she had really faced to get to where she was.”

In 1979, after doing research at CERN for 15 years, Dr. Gaillard began earning a partial salary. The following March, on International Women’s Day, she disseminated a 36-page report on the status of female researchers at CERN, who made up only about five percent of the institution’s scientific personnel at the time.

Dr. Gaillard left CERN in 1981 after she was passed over for a senior staff position. She had accepted an offer to be a tenured professor in the physics department at Berkeley — the first woman to achieve that distinction.

Dr. Gaillard’s appointment showed that physicists did not have to be arrogant or aggressive to succeed, said Marjorie Shapiro, who was then a physics graduate student at Berkeley and who in 2004 became the first woman to chair the physics department at Berkeley. For her, Dr. Gaillard’s professorship “was always an important model,” she said.

Dr. Gaillard led the particle theory group at Berkeley Lab. She spent the later part of her career working on supersymmetry, a theory that could account for physical observations not described by the Standard Model.

As a member of the National Science Board, she helped secure federal support for the next generation of particle experiments. She also furthered efforts to make the field more equitable for women. Dr. Gaillard was inducted into the National Academy of Sciences in 1991.

“She opened the door for many of us,” said Belén Gavela, a professor at the Autonomous University of Madrid who studied under Dr. Gaillard in 1980. Nine years later, Dr. Gavela became the first female staff scientist hired by CERN’s theory division.

Dr. Gaillard retired in 2009, but she continued to work on physics problems every day at her home in Berkeley, where she died, her son Bruno said.

Dr. Gaillard’s first marriage ended in divorce. In 1984, she married the physicist Bruno Zumino, who died in 2014. In addition to her son Bruno, she is survived by two other children, Alain and Dominique Gaillard, and seven grandchildren.

As a teenager, Dominique Gaillard helped distribute the report on female scientists at CERN, but she did not fully realize that her mother was a trailblazer until she moved to Berkeley, she said. The example left an impression, regardless.

“I grew up not really aware of those barriers,” Ms. Gaillard, who works in tech, said in an interview. “I followed in her footsteps. It never occurred to me that I couldn’t be as good in a room of men.”