Former mayor Thomas M. Menino has been diagnosed months after leaving office with an advanced form of cancer that has spread to his liver and lymph nodes, a jolting discovery that he described as the most profound challenge of his life.

Menino has never been one to hide his feelings, not the joy he felt at governing his city for a record 20 years, not the agony over retiring from the job he loved. But as he sat in his elaborately paneled university office Friday in a quiet corner of Boston, he concealed his emotions with a steady determination.

His doctors described in a separate interview an advanced cancer of unknown origin that had metastasized. Despite an exhaustive search, doctors said they have been unable to find the source of the cancer and suggested they never will. It was discovered in early February as his primary care physician, Dr. Charles A. Morris, examined Menino’s back because of chronic weakness in his legs.

“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. .....He knew he could not keep the frenetic pace of his old schedule. “You need to be in the neighborhoods all the time,” Menino said. “That’s what a mayor is supposed to do, keep the pulse of the community.”

The cancer diagnosis came just weeks into his new job at BU. He acknowledged his surprise, but remained determined.