Former President Jimmy Carter has defied odds and is officially 100 years old today — and he credits his longevity to the strong relationships in his life, especially his marriage.

James Earl Carter, Jr., more commonly known as Jimmy Carter, served as the 39th U.S. president from 1977 to 1981. He's outlived all presidents in U.S. history, earning the distinction of being the oldest former president. 

In 2019, Carter told People Magazine that he never expected to live as long as he has: "It's hard to live until you're 95 years old."

In February of 2023, President Carter entered home hospice care after several hospital stays over the years, following his cancer diagnosis in 2015.

"I just thought I had a few weeks left, but I was surprisingly at ease," he said during a press conference in 2015. "I've had a full life, I have thousands of friends … so I was surprisingly at ease, much more so than my wife was."

What's his secret to living a long and happy life? Who he married. He and his wife, Rosalynn Carter, who died in November 2023 at 96 years old, kept each other going.

When asked what he believed helps people live longer, he said to People: "I think the best explanation for that is to marry the best spouse: someone who will take care of you and engage and do things to challenge you and keep you alive and interested in life."

Married for 77 years, the former president and first lady didn't have longevity in mind, he told People; they were simply looking for ways to serve their community and pour into each other.

Their love story began when the two met at Plains United Methodist Church in their hometown of Plains, Georgia, Mr. Carter told The Washington Post. He asked her on a date to the movies, and told his mom that he was going to marry Rosalynn the next morning, he said in his interview with The Post.

They went on to share four children and a total of 22 grandchildren and great-grandchildren.

The two also founded The Carter Center, a nongovernmental organization devoted to improving lives, in 1982. Since then, the center has built thousands of homes in more than ten countries alongside Habitat for Humanity.

At the same time, the Carters were making incredible memories together through activities like tennis, downhill skiing and bird-watching: "Rosa and I have seen about 1,300 different species of bird," he said in his interview with People.

After his presidency, the dynamic duo moved back to their hometown and purchased a modest $167,000 home — which is a much lower cost compared to the median home price in Georgia, Zillow's data shows.

"Now when we have a quiet moment, like a birthday or something, we like to stay at home, just by ourselves," he added, "And enjoy a quiet day in our own house without any visitors and with minimum phone calls and emails coming in."

The Carters celebrated their 77th wedding anniversary in July 2023 at their home in Georgia, making them the longest-married first couple.

"As we have looked back at their legacy, it has been really wonderful to see the outpouring of support and respect and love," their grandson Jason Carter said in 2023.

Want to master your money this fall? Sign up for CNBC's new online course. We'll teach you practical strategies to hack your budget, reduce your debt, and grow your wealth. Start today to feel more confident and successful. Use code EARLYBIRD for an introductory discount of 30% off, now extended through September 30, 2024, for the back-to-school season.

Plus, sign up for CNBC Make It's newsletter to get tips and tricks for success at work, with money and in life.