WHEN LINDA MASON was growing up in the 1950s and ’60s, her hometown, Homer, New York, wasn’t exactly a gateway to the world. Her father, Sam, had chosen this quiet dairy farming community far north of New York City to open a medical practice. He became the small-town physician who knew everyone’s name, making house calls until the day he retired.

Sam Mason, though, had a sense of duty and adventure that took him well beyond Homer. He occasionally put the practice on hold and traveled the globe to volunteer his medical skills to needier places such as Honduras, Guatemala, and the Democratic Republic of Congo (then known as Zaire).

Although Linda Mason didn’t go on those trips, she was captivated by the stories he brought back. One year he even returned with a Congolese nurse, who lived with the family for a time. Her father, through his experiences, became her window abroad. “It just had a profound impact on me,” she says.

For decades, Mason, who is 57 and lives in Belmont, has built on his example. Since cofounding child care company Bright Horizons Family Solutions in 1986, Mason has become a luminary in humanitarian work. Now board chairwoman of the worldwide relief organization Mercy Corps, she and her organization have documented atrocities in the Darfur region of Sudan and aided civilians in Congo, a place she once described as “ripped apart by brutality and lawlessness.” They helped Indonesians rebuild huts and fishing boats after the 2004 tsunami. And they trained adults to comfort traumatized children in Haiti after its 2010 earthquake. When disaster calls — and it frequently does — Mason often answers.

In each place, those who have known and worked with her say, the mother of three displays entrepreneurial spirit, a big heart, and steely grace. Her calm demeanor, they say, obscures a gritty determination and fearlessness. “You can’t travel with everyone to a war zone, but you can travel with Linda Mason,” says the Rev. Gloria White-Hammond, a Boston physician, activist, and minister who accompanied her to Darfur in 2005.

Mason’s first true taste of international aid work came after she earned her MBA from Yale in 1980. That summer, she and two friends — including Roger Brown, who would later become her husband — traveled to the Thai-Cambodian border to help manage camps for refugees reeling from the deadly reign of the Khmer Rouge. Many Cambodians had no means of supporting themselves. “They did not have enough food,” says Phlong Than, a Cambodian with whom Mason and Brown worked and who is now 73 and living in Oregon. (Than features prominently in the 1983 book Mason and Brown co-wrote about their nearly yearlong stint, Rice, Rivalry, and Politics: Managing Cambodian Relief.) “I just never saw the world the same after that experience,” Mason says.

After a few years working in management consulting in the United States, she and Brown went back overseas to build a relief program for Save the Children in Sudan, which, like other countries in sub-Saharan Africa, was hit hard by famine in the mid-1980s. The conditions were tough — there’d been a coup, and they worked in 120-degree heat and had little contact with the States — but Mason and Brown stayed on for the better part of two years, returning in the summer of 1986. Exhausted, they sought a new social mission, but closer to home.

That mission became Bright Horizons, a for-profit company whose innovations — establishing top-quality child care centers at big employers and treating employees far better than the industry norm — would earn them wide acclaim. Soon after founding the company in their Cambridge kitchen — with some help from partners, including Bain Capital, then led by Mitt Romney — Mason and Brown launched a companion nonprofit called Horizons for Homeless Children, to help struggling parents find a pathway to economic stability. Bright Horizons, which Bain Capital took over in 2008 in a $1.3 billion deal, now serves some 150,000 children at 750 locations. Mason and Brown (who together held $4.6 million worth of Bright Horizons shares at the time of the acquisition) remain on the company board, Mason as chairwoman.