The former US president appeared upbeat and made jokes as he told a news conference yesterday that he was to begin radiotherapy treatment immediately.
He said he will cut back “fairly dramatically” on his work at the Carter Centre foundation.
The former president added: “I’m ready for anything and looking forward to a new adventure.”
Mr Carter, 90, said a tumour showed up on an MRI scan after he sought treatment at Emory Hospital in Atlanta.
He said he first felt sick while he was in Guyana in May to monitor an election. He added that he underwent surgery to remove the tumour on his liver on 3 August.
Mr Carter said: “I get my first radiation treatment for the melanoma in my brain this afternoon.”
Wearing a dark blazer, red tie and jeans and surrounded by friends and family, he said at first he thought the cancer was confined to his liver and that the operation he had earlier this month had completely removed it, “so I was quite relieved”.
But that same afternoon, the MRI showed it was on his brain.
“I just thought I had a few weeks left, but I was surprisingly at ease,” he said.
“I’ve had a wonderful life.”
Mr Carter said about a tenth of his liver was removed on 3 August, before four spots of melanoma were found on his brain.
He added that no cancer has been found on his pancreas or any other part of his body so far, and doctors are monitoring him closely.
He said he is not feeling despair or anger over his health, and feels good, with only slight pain.
Mr Carter said that if he does not make his scheduled trip to Nepal in November, others from his family will probably go in his place.
His father, brother and two sisters died of pancreatic cancer.
Mr Carter was the US’s 39th president, defeating Gerald Ford in 1976. Ronald Reagan succeeded him in 1980 after a landslide victory, and Mr Carter founded the Carter Centre in 1982 to focus on global health care and democracy.
The native of tiny Plains, Georgia, rebuilt his career as a humanitarian guiding the centre focused on global issues, including health care and democracy.
Mr Carter earned a Nobel Peace Prize in 2002, helped defuse nuclear tensions in the Koreas and helped avert a US invasion of Haiti.
He and his wife, Rosalynn, still make regular appearances at events in Atlanta and travel overseas.
When the couple are in Plains, Mr Carter frequently teaches a Sunday School class before services at Maranatha Baptist Church.
He plans to teach this weekend as scheduled, according to the church.
“No matter where we are in the world, we’re always looking forward to getting home to Plains,” Mr Carter said, adding that he hasn’t felt any weakness or debility..