The former US president said at a news conference said he was starting radiotherapy immediately.
He said he will cut back "fairly dramatically" on his work at the Carter Centre foundation.
Mr Carter 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 August 3.
"I get my first radiation treatment for the melanoma in my brain this afternoon," Mr Carter said.
But the committed Christian added the diagnosis would not stop him from teaching Sunday school classes at church.
He said he would continue to take lessons in his home town of Plains, Georgia, for as long as he can.
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. I've had a wonderful life," the 90-year-old said.
Mr Carter said about a 10th of his liver was removed on August 3, 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.
"I'm perfectly at ease with whatever comes. I'm ready for anything. I'm looking forward to a new adventure."
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 rebuilt his career as a humanitarian, founding the Carter Centre in 1982 to focus on global health care and democracy. He won the Nobel Peace Prize in 2002.