Photographer Mary Ellen Mark (Ralph Gibson)

Mary Ellen Mark and her camera went to places most of us would never dare. Since the 1960s, she photographed the outcasts of the world: runaway kids, prostitutes in India, women in a prison for the mentally ill.

All of her subjects, no matter how down and out, looked straight back at Ms. Mark, and her camera never flinched. With one fearless project after another, she became one of the preeminent documentary photographers of our time. Her work regularly appeared in the pages of Vanity Fair, Rolling Stone, Esquire and Vogue.

Still in the prime of her career, Ms. Mark died May 25 in New York City at age 75. Her Web site and official representatives confirmed her death, but details concerning the cause were not immediately disclosed.

In one of her first documentary projects, Ms. Mark went to London in the 1960s to photograph the distinctly unglamorous lives of heroin addicts, and from then on she was drawn to exploring life on the margins.

“I feel an affinity for people who haven’t had the best breaks in society,” she told the New York Times magazine in 1987. “I’m always on their side. I find them more human maybe.”

She spent 36 days in a maximum-security ward in Oregon in the 1970s, chronicling the lives of mentally ill women.

The resulting images, critic Robert Hughes wrote in Time magazine in 1978, constitute “one of the most delicately shaded studies of vulnerability ever set on film.” Her pictures were marked by unusual angles, stark contrasts in texture and lighting and vivid figures emerging from murky backgrounds.

Throughout her career, Ms. Mark conveyed a deep humanity in her black-and-white photographs, but her gaze was not one of pity. She traveled often to India, where she spent months earning the trust of the prostitutes of Mumbai.

“Every day I had to brace myself for the street as if I were about to jump into freezing water,” she told Britain’s Guardian newspaper in 1991. “I started out by just walking the street. It was the same as always, crowds of men, women alternately hurling insults and garbage at me . . . Some of the women thought I was crazy, but a few were surprised by my interest and acceptance of them. Very slowly, I began to make friends.”

In time, they came to look protectively on Ms. Mark and once, during a police raid, hid her under a bed. Her 1981 book “Falkland Road” captured the alien, oddly alluring world with all its squalor, color and pathos. (It was also one of the few times Ms. Mark worked in color film.)

In the United States, Ms. Mark ventured into hospices to portray the dying. She spent time with autistic children. She stepped into the world of white supremacists, photographing members of the Ku Klux Klan. She found the Damm family in a homeless shelter in Southern California in the 1980s. She later showed them living in a car, the parents sprawled in the front seat, the children looking out the back window at a bleak landscape.

For all the heartbreak and hopelessness in her images, Ms. Mark did not depict her subjects as oddities or specimens. They were merely other human beings driven by circumstance to the ragged edges of society.

Many of Ms. Mark’s most remarkable images came in a series of photographs of runaway children in Seattle in the early 1980s. Pictured: "Rat" and Mike with a gun, Seattle, Washington, 1983. (Mary Ellen Mark)

Many of Ms. Mark’s most remarkable images came in a series of photographs of runaway children in Seattle in the early 1980s. In a city called the country’s “most livable,” she discovered an underground culture of feral children. Abandoned or simply forgotten, they lived on their wits, eating out of garbage cans, taking drugs and brandishing guns. A 13-year-old girl named Tiny was being used in prostitution when she wasn’t getting high. (When Ms. Mark photographed her again at the age of 30, Tiny had five children by five different men.)

After gaining their trust, Ms. Mark portrayed the Seattle children her 1988 book “Streetwise.” A related documentary film of the same name, made by Ms. Mark’s husband, Martin Bell, was nominated for an Academy Award in 1984.

“One picture by Mary Ellen Mark is almost too many, yet a hundred of her photographs is not enough,” Washington Post writer Hank Burchard said in 1994. “Few photographers have ever had both the visceral impact and the intellectual depth of Mark.”

Mary Ellen Mark was born March 20, 1940, in Philadelphia. Her father was an architect.

She graduated from the University of Pennsylvania in 1962 and stayed on at Penn for a master’s degree in photojournalism two years later.

She was drawn to the photography of Robert Frank, W. Eugene Smith and Diane Arbus and began traveling around the country with a camera in graduate school.

“I became obsessed by it,” she said in 1987. “I knew immediately it would be my life’s work. I knew I had a chance of being good at it.”

Along with Arbus and her contemporary, Annie Leibovitz, Ms. Mark became one of the most renowned female photographers in a largely male field.

She published the first of her 18 books in 1974 and financed her photojournalism projects by taking still photographs on the sets of movies, including “Apocalypse Now” and “The Missouri Breaks.” Later, as magazines began to publish fewer photographs, she took assignments for advertising and celebrity portraits. Her photographs are in the collections of many major museums.

At the time of her death, she was photographing New Orleans a decade after Hurricane Katrina. She was also scheduled to lead photography workshops in New York and Mexico in the coming months.

Survivors include her husband, of New York.

Ms. Mark professed to know little about the mechanical side of photography and used a variety of cameras throughout her career. She continued using only film long after digital cameras became the norm.

Her photography advanced no political views and she avoided the journalistic convention of narrative resolution.

“I just go into it trying to get good pictures,” she said.