Nathan Reynolds was driving to his hotel in Marina, California, in January 2015, when he saw a car blow through an intersection and careen down a nearby embankment into a retention pond.

Mr. Reynolds, a 37-year-old hydrogeologist and Claremont resident, didn’t wait for help to arrive. After rushing to the bank and spotting the car partially submerged in the water, he acted quickly.

“I could see somebody waving their hands frantically in the back seat,” he recalled. Mr. Reynolds didn’t think twice; he yelled at a pedestrian to call 9-1-1, stripped off his work boots, emptied his pockets and dove into the deep, frigid pond.

“I was just concerned with the little time I had and trying to get them out,” Mr. Reynolds said. “I didn’t want to put anybody else at risk, so I just did what I thought was right.”

After swimming out to the car, he saw an elderly woman, 75-year-old Olivia Schreiber, in the front seat, and her 9-year-old great-granddaughter Aislinn Crooks in the back. Mr. Reynolds motioned for Ms. Schreiber to roll down her window, rather than risk submerging the car further by opening a door.

“I learned that she had passed out at the wheel; she didn’t know why,” he said. “I also found out that neither of them knew how to swim. They were basically in a sinking car in the middle of the pond and we had to figure out a way to get them to the shore.”

Thinking quickly, Mr. Reynolds had Aislinn crawl into the front seat and out the window. She wrapped her arms around his neck and he swam her to shore.

“At that time, people had started lining up along the bank,” Mr. Reynolds said, “but I had to yell for somebody to come down and help, so I could pass her off to them.

“I looked back at the car and it was sinking, it was getting closer, and I was worried I wasn’t going to be able to get there in time,” he added.

Exhausted, swimming in jeans, Mr. Reynolds made it back just in time, and helped Schreiber free herself from her seatbelt, which was stuck.

“Pretty much just as she got out of the car,” he remembered, “the car went completely below the surface of the water.”

Mr. Reynolds tried to swim her to shore, where police had arrived and were beckoning him, but said Ms. Schreiber must have been in shock and was fighting him.

“I was underwater and I tried to push her up to the officers and I think they saw that I was struggling,” he said, “and at that point they came into the water and helped me get her the rest of the way out of the water.”

Mr. Reynolds, who was in Marina working on a desalinization project, said the reality of the situation didn’t register until later. A local news station came to Aislinn’s school, and Mr. Reynolds watched footage of her classmates drawing notes saying they were glad she was alive, and thanking him for his heroic action.

“The gravity of it hit me once I saw that, and I was like, ‘Wow, this really happened and they’re alive and I was responsible for that,’” he said. “It was really special.”

This April, Mr. Reynolds’ life-saving deed was honored with a medal from the Carnegie Hero Fund Commission, an organization founded by famous industrialist, businessman and philanthropist Andrew Carnegie in 1904.

The fund recognizes civilians who act with extraordinary bravery in saving people’s lives, often at the cost of their own.

Mr. Reynolds was recognized, fittingly, at Pomona College’s Carnegie Hall, which was originally a library funded by Carnegie himself in 1905.

Linda Hills, Carnegie’s great-granddaughter, a member of the Hero Fund Commission’s board and a 1969 graduate of Pomona College, was on hand to present the award.

“Nathan is absolutely the most gracious young man,” Ms. Hills said. “It was really an honor to be able to present this to him. It’s only the second one that I’ve personally presented.”

Hills said the courage and valor Carnegie Medal awardees demonstrate is remarkable.

“We read these various cases and it is incredible to me the number of people who actually freeze,” Ms. Hills said. “They say, ‘Oh my god, what are we going to do?’ But they don’t act. But there is something, a spark inside these people who are able to act and go out and risk their lives for people.

“Ninety-nine percent of the time they don’t know these people,” she added. “They’re complete strangers, but (these heroes are) compelled by something inside of them to go out and try to save this person’s life.”

Mr. Reynolds, who was honored in front of his family and friends, described the ceremony as a “surreal experience.”

“It’s the great-granddaughter of Andrew Carnegie coming to give me a Carnegie medal at, of all places, Carnegie Hall down the street from my house,” he said. “It’s just a huge honor to get that award. It’s just a really special, special thing.”

—Kellen Browning