For any person, celebrating a century of life is a rare milestone. For Jimmy Carter, the nation's 39th president, such longevity is an exclamation point on an unparalleled life of public service.

Some might assert that the presidency itself is the pinnacle of public service. But Carter's exit from the White House after what many critics decried as a largely ineffectual single term marked the beginning of what became the most consequential post-presidency in U.S. history – made even more remarkable by its inauspicious start.

Jimmy Carter had lost his 1980 bid for reelection by ten percentage points, pulling just 41% of the popular vote versus 51% by his Republican challenger, Ronald Reagan. Shortly afterward, he discovered that the prosperous agricultural business he had built earlier in his career had been driven into the ground by a blind trust, leaving him millions in debt, adding to the $1.4 million of debt he had accrued as part of his failed reelection campaign, with no cash reserves to pay it off.

Then there was the unresolved matter of the Iranian Hostage Crisis, which had crippled Carter's presidency, leading critics to charge him with being weak and ineffectual. When Reagan succeeded him as president on Jan. 20, 1981, Carter had been awake for over two full days as he oversaw the negotiation to release the 52 American hostages that had been detained by the Iranian government for 444 days. A last thumb in Carter's eye: They would be freed in the first minutes of Reagan's presidency.

Upon relinquishing the White House, Carter returned briefly to his native Plains, Georgia, which at the time boasted a population of just 640 residents, where he was welcomed home in a driving rain by friends and neighbors with a covered-dish supper. The rain-soaked homecoming didn't last for long. Just hours afterward, Carter flew off to Wiesbaden, Germany, to greet the freed hostages, only to be met with anger by many of them who believed he had failed to ensure their liberation earlier.

On Jan. 22, 1980, Carter returned to Plains, to the ranch-style home he and wife Rosalynn had built in 1961, but hadn't lived in for ten years. Exhausted and depleted, the now former president slept for 24 hours before awakening to what he described as "an altogether unwanted life" – and with no idea what he would do next.

Twenty-one years later, Carter would be awakened by an early-morning phone call in that same home, in that same small town, with the news that he had won the Nobel Peace Prize for, as the Nobel committee wrote, "his decades of untiring effort to find peaceful solutions to international conflicts, to advance democracy and human rights, and to promote economic and social development."

Along the way, Carter reinvented the post-presidency, manifesting its possibilities and potential and providing a playbook, and a daunting standard, for activist former presidents. He showed how a president can leverage the stature of being a "former" to advance a philanthropic agenda, while enhancing his overall legacy and strengthening the American brand.

The improbable journey from newly defeated one-term president to Nobel laureate reflected a pattern throughout Carter's fruitful life, one spent achieving outsize ambitions by defying long odds. Carter launched his career in politics in 1962 by challenging the political machine in Southwestern Georgia, successfully contesting a rigged election and winning a seat in the Georgia state legislature. While he lost a race for Georgia governor four years later, he came back to win the office in 1970, becoming one of a crop of new leaders to usher in a new, post-segregation South.

After leaving the governor's mansion in 1975 due to a state law then prohibiting governors from serving consecutive terms, Carter set his sights on the distant presidency, a dark-horse former governor of a deep Southern state with little or no name recognition – so much so that even his home state’s Atlanta Constitution newspaper ran a story headlined, “Jimmy Who Is Running for What?”

“Nobody thought I had a chance in God’s world to be the nominee,” Carter told me in 2013.

His unlikely nomination as the Democratic presidential candidate and subsequent victory over incumbent President Gerald Ford in the 1976 election spoke to Carter's relentless drive and preternatural self-assuredness.

Those same qualities would come to bear in his post-White House endeavors. Just a little more than a year out of office, as he considered his future, Carter had an epiphany: that he could create a nongovernmental, non-profit organization that could focus on intractable problems around the world which the international community and the United Nations were not addressing. The Carter Center, attached to his presidential library in Atlanta, did just that, becoming an outlet for the former president's activism and vision for a better world.

Since its launch in 1982, the Carter Center, scrupulously overseen by Carter himself, has monitored over a hundred elections in 39 countries and has helped to peacefully resolve disputes throughout the world – including in Haiti, Sudan, and Bosnia – while working toward the eradication of Guinea worm disease and river blindness, insidious ailments that went largely unchecked among the world's poor and developing nations. Recognizing his skill in conflict resolution, President Bill Clinton tapped the ex-president in the 1990s to represent the U.S. in a negotiation to dismantle North Korea's nuclear weapons program, and in staving off a U.S. military invasion of Haiti.

As prodigious as they were, Carter's activities weren't limited to the Carter Center. Soon into his post-presidency, Carter took up a hammer for Habitat for Humanity, providing both labor and inspiration for the next four-and-a-half decades on work projects that bear his name. Somehow, he also found time to teach Sunday school nearly every week at Plains' Maranatha Baptist Church – and pose afterward for photographs with visitors to the congregation – as well as do woodwork, fly fish, paint, and become our most prolific presidential author.

Carter sometimes bristled when he was called "our best ex-president," a backhanded compliment that disregarded a presidential term that he saw as largely successful. "I don't know of any decisions I made in the White House that were basically erroneous," he told me in 2005.

But he didn't spend a great deal of time worrying about his place in the presidential pantheon. The things Jimmy Carter wanted to be remembered for went beyond any achievement he may have chalked up in the White House.

In 2014, in an interview at the LBJ Library, I asked Carter how he wanted to be remembered.

"I think lot of people will say 'He only served one term and got defeated [for reelection],'" he replied. "I would like for people to remember that I kept the peace and that I promoted human rights …That would be my preference."

He will get his wish.

Mark K. Updegrove is a presidential historian and ABC News political contributor. He is the president and CEO of the LBJ Foundation, and the author of "Second Acts: Presidential Lives and Legacies After the White House."