Menino, 71, was diagnosed in early February -- just a month after leaving office -- with cancer that has spread to his liver and lymph nodes, the Boston Globe reported Sunday.
Dr. Charles A. Morris, Menino's primary care physician, discovered the cancer while giving the former mayor an exam.
"They did a scan and Morris called me and said, 'You have to come see me.' He was more shaken up than I was," Menino said Friday.
Menino began an intravenous chemotherapy regimen in early March at Dana-Farber Cancer Institute.
"My attitude really is, we'll get through it," Menino said. "We got through the [illnesses in 2012], we'll get through this. I have great doctors and supportive friends."
Morris said the new diagnosis is unrelated to Menino's two previous bouts with cancer. In 2003, Menino underwent surgery to have a cancerous growth removed from between his shoulder blades, and in 2012, he had a cancerous growth removed from his nose.
"There would be no connection between this and his other medical problems," Morris said. "And no connection with his medical issues that were publicly discussed over the course of the last year."