For much of the year, Dr. Darin Portnoy works in near obscurity at a health clinic in the Bronx.

But in the last decade, the 45-year-old physician has spent months at a time in war-torn and developing countries, providing medical care to those who find themselves in the throes of crisis — whether that crisis is a regional health epidemic or civil strife.

Since 1997, Dr. Portnoy has been a field doctor with Doctors Without Borders, traveling to Uzbekistan, El Salvador, Georgia, Sudan, and other countries. In 2004, he became president of the international group’s board of directors in America.

In the field and as a top American administrator for the group, also known as Médecins Sans Frontières, he has seen things most of his colleagues in the Bronx cannot fathom, including a measles epidemic in Nigeria and a hospital that was leveled and needed rebuilding in Liberia. Most recently, Dr. Portnoy returned from Darfur, where he worked in the emergency and pediatric departments at a hospital.

Seated in a conference room at the group’s Manhattan offices earlier this month, Dr. Portnoy discussed the organization’s work in 70 countries worldwide. He was dressed casually, in jeans and a turtleneck sweater, with a messenger bag on the floor near his feet.

Dealing with violence is a daily reality for the organization, and Dr. Portnoy stressed the nuanced position of providing humanitarian aid amid divisive conflicts. “As long as they leave their weapons outside the door, we treat people on both sides of armed conflicts,” he said.

As a top administrator for Doctors Without Borders in America, Dr. Portnoy brings the “field mentality” of doctors to the boardroom, where he helps to develop strategy and to set priorities for the group, such as the prevention of sexual violence and the transmission of AIDS between mothers and children, and the use of therapeutic food with malnourished patients.

He also represents the American office on an international level, a position that often requires him to explain America’s foreign policy to his colleagues. “I get asked a lot about that,” he said.

Dr. Portnoy, speaking in measured tones, stressed that the group is not political. It does not accept funding from the United Nations, and it does not participate in military operations because it does not want its volunteers to be perceived as combatants.

“We just speak about what we see,” he said, adding, “We’re often the first ones there.”

“I think that’s something any aid organization can and should do,” he said.

Doctors Without Borders was founded in 1971 by a group of French journalists and doctors, including Bernard Kouchner, who later left the organization and is now France’s foreign minister. Currently, the group operates out of 19 offices around the world. None of its volunteers are armed, but volunteers are protected by armed guards in Somalia.

Visibly upset, Dr. Portnoy discussed the recent murder of several colleagues in Somalia, a country deemed by the organization as too dangerous for its volunteers to be unarmed.

“It’s very difficult to return to this and not think about it,” he said.

Since 2005, Dr. Portnoy has lived in New York, where he is an attending physician at Montefiore Medical Center.

Here, his patients are a diverse group, considered by domestic standards to be medically underserved. Routine ailments include diabetes, hypertension, and heart problems — a far cry from the illnesses he sees in the field, such as respiratory illness and malnutrition.

“We’re taking care of the most vulnerable,” he said, referring to his fieldwork. “It’s not the same sort of well child visit” that he sees in the Bronx. If there is any overlap, he said, it is because some current patients moved here from countries where he has worked.

Dr. Portnoy was born in West Virginia; his father is a Juilliard-trained violinist and conductor and his mother is a professor of speech communication. Dr. Portnoy said he chose to become a doctor because medicine offered a “concrete” path to changing a person’s condition.

His first brush with Doctors Without Borders came in the 1980s, when he was a student at Tulane University. A student of international relations, he received a grant to conduct research in Honduras, and he met a group of volunteer doctors on the airstrip when he landed. In 1989, Dr. Portnoy graduated from Tulane with a medical degree and a master’s degree in public health. He spent the next four years doing a residency in family medicine at John Peter Smith Hospital in Fort Worth, Texas.

Subsequent jobs have taken him across the country, to New Mexico, Alaska, and Colorado.

He joined Doctors Without Borders in 1997 as a field doctor and later as a field coordinator for tuberculosis treatment and control programs in Uzbekistan.

Several years ago, Dr. Portnoy met his wife, a television producer, when she was working on a 13-part series about Doctors Without Borders for National Geographic. He was not part of the series.

In June, Dr. Portnoy will conclude his term as president of the board, but he plans to accept future field assignments to build on his previous work. “It’s hard not to be affected by the things that you see,” he said.