From childhood, Peter Buck remembered an Italian-style sandwich shop called Amato’s near his home in Maine. The long lines outside the door made a strong impression, as did the diamond rings and gold bracelets worn by women behind the counter.

By 1965, Dr. Buck was a physicist who worked on designs for nuclear power plants. When 17-year-old Fred DeLuca, the son of a friend, asked him for advice on how to pay for college, Dr. Buck remembered those sandwiches. He lent the young man $1,000, and they became partners in a sandwich...

From childhood, Peter Buck remembered an Italian-style sandwich shop called Amato’s near his home in Maine. The long lines outside the door made a strong impression, as did the diamond rings and gold bracelets worn by women behind the counter.

By 1965, Dr. Buck was a physicist who worked on designs for nuclear power plants. When 17-year-old Fred DeLuca, the son of a friend, asked him for advice on how to pay for college, Dr. Buck remembered those sandwiches. He lent the young man $1,000, and they became partners in a sandwich shop called Pete’s in Bridgeport, Conn. Through aggressive franchising, it grew into today’s Subway chain, with nearly 40,000 locations world-wide.

Dr. Buck, who died Thursday at the age of 90, maintained a low profile but had a fortune of $1.7 billion at the time of his death, Forbes estimated.

He was born Dec. 19, 1930, in South Portland, Maine, and grew up on a farm producing lettuce, squash and celery. His mother, Lillian “Molly” Buck, worked as a journalist and prodded her two sons to go to college.

At Bowdoin College in Maine, he studied economics and received an undergraduate degree in 1952. He then enrolled at Columbia University, where he earned master’s and doctoral degrees in physics. During his career as a nuclear physicist, Dr. Buck worked in laboratories for General Electric Co. and United Nuclear Corp. in New York state and for Nuclear Energy Services in Danbury, Conn.

His investment in Subway wasn’t an immediate success. “We didn’t make a profit for 15 years,” Dr. Buck told The Wall Street Journal in 2014.

Today, Subway has nearly 40,000 locations world-wide.

Photo: Richard B. Levine/Zuma Press

His survivors include two sons and five grandchildren. His marriage to Haydee Piñero Buck —the daughter of Jesus T. Piñero, a former governor of Puerto Rico—ended in divorce in 1977 after 22 years. He then married Carmen Lúcia, a native of Brazil who came to the U.S. to study. She died in 2003.

In 2004, he donated a 23.1-carat Burmese ruby, called the Carmen Lúcia Ruby, to the Smithsonian’s National Museum of Natural History. He also made donations to Danbury Hospital, Bowdoin College and charter schools in Connecticut and New York. He established the Tall Timbers Trust, which held more than 1.2 million acres of timberland in Maine, according to Subway.

“I don’t give back,” he said of his philanthropy. “I give.” He credited his wife Carmen for spurring many of his gifts. When she offered an idea for a donation, Dr. Buck said, he typically responded, “Yes, dear.”

He spent some of his money on a hobby of flying glider planes and on a ranch in Brazil.

When a Wall Street Journal reporter interviewed him seven years ago, he was living in Danbury and driving a 17-year-old car. He said he ate at least five Subway sandwiches a week.

When asked whether he was a billionaire, Dr. Buck replied, “Yeah, I guess so.” Even so, he said, “I can go any place in Danbury and nobody recognizes me.”

Write to James R. Hagerty at bob.hagerty@wsj.com