Doctors removed about one-tenth of his liver during the surgery, which he said healed quickly and left him with "minimal pain."
He will undergo a single course of radiation treatment -- though more may be necessary, in the future -- and four treatments with an immune system-boosting drug, which will be administered at three-week intervals.
Carter said doctors will continue to scan his body for cancer, in an effort to determine where the melanoma originated.
As Carter noted, while in a majority of cases melanoma is a cancer of the skin, a relatively small portion of cases occur inside the body. Rarely, it can develop in places other than skin, such as the eye or mucous membranes.
According to Memorial Sloan Kettering Cancer Center, "one of the most common primary tumors to spread to the brain is malignant melanoma. In nearly 50 percent of people with melanoma that has metastasized, the disease can be found in the brain."
The center added that "the outlook for patients with brain metastases generally depends on the number, size, location, and origin of the primary tumor or tumors."
Carter will be receiving a drug called Keytruda to boost his immune system. Keytruda became available only in the last year and is so new that it is unclear whether it might improve his chances of survival with the cancer.
In general, the average length of survival with melanoma is 11 months, if surgery and radiation do not successfully remove the lesions or tumors, The Washington Post's Amy Ellis Nutt reported.
In May, during a trip to Guyana to monitor elections there, Carter fell ill with a "very bad cold" and returned to Atlanta early. Doctors performed a complete physical examination and found the tumor on his liver.
By the end of June, doctors were certain that he would need to have an operation on his liver, Carter said. But he chose to delay surgery in order to complete a book tour for his memoir published this year, "A Full Life: Reflections at Ninety."
"The doctors told me that it was a very slow growing cancer, apparently it wouldn't make any difference between the middle of July and August, so we scheduled it when I got through with the book tour," Carter said.
Carter said that an MRI scan the afternoon after the cancerous mass was removed from his liver revealed that melanoma had already spread to his brain.
"I just thought I had a few weeks left," he said during the news conference at the Carter Center in Atlanta. "But I was surprisingly at ease -- much more so than my wife was."
The former president will be treated at the Winship Cancer Institute, about two miles from the Carter Center. Both the institute and Carter's organization are affiliated with Emory University.
Carter is the second-oldest living president, separated by just over 100 days in age from George H.W. Bush. Carter, the 39th president, served in the White House from 1977 to 1981.
After his presidency, he has continued his humanitarian and human rights work through the Carter Center. It was for that work that he was awarded a Nobel Peace Prize in 2002.
For now, he noted that his cancer treatment will become his "top priority."
"The Carter Center is well prepared to continue on without any handicap," Carter noted.
Several members of Carter's immediate family -- his brother, two sisters, his mother and father -- have all died from cancer.
Beyond his international travel schedule, Carter and his wife of 69 years, Rosalynn, have maintained a life in Plains, Ga., his hometown. He noted that he doesn't expect cancer treatment to halt his usual contributions to his place of worship, Maranatha Baptist Church.
"I plan to teach Sunday school this Sunday -- and every Sunday as long as I'm physically and mentally able," Carter said.
He noted that the well wishes have poured in from President Barack Obama and the first family, as well as all of the living former presidents.
I spoke with President Carter to wish him well, and he sounded strong. Bar and I are wishing him the very best as he fights the good fight.— George Bush (@GeorgeHWBush) August 20, 2015
President Carter is as good a man as they come. Michelle and I are praying for him and Rosalynn. We're all pulling for you, Jimmy.— President Obama (@POTUS44) August 20, 2015
Former secretary of state Hillary Rodham Clinton and current Secretary of State John F. Kerry have also called to wish him well.
"It's the first time they've called me in a long time," Carter joked.
Carter spoke to a full room of journalists, reflecting on -- and occasionally joking about -- his legacy in the White House and as a global humanitarian.
He called his work with the Carter Center "personally more gratifying" than his presidency, though he joked that he wished he might have been able to serve a second term and then engage in humanitarian work after leaving the White House.
"I think I have been as blessed as any human being in the world," Carter said. "So I'm thankful and hopeful."
This story has been updated to clarify that Carter misspoke when he said during his press conference that he would receive four radiation treatments. The Carter Center said in a statement late Thursday that he would receive one radiation treatment and four treatments with the immune-boosting drug Keytruda.
Amy Ellis Nutt contributed to this report.