The Rochester, Minn.-based clinic specified his condition as Bipolar II, which is defined as periodic episodes of depression and hypomania, a less serious form of mania.
"Congressman Jackson is responding well to the treatment and regaining his strength," the clinic said in a statement.
Bipolar II is a treatable condition that affects parts of the brain controlling emotion, thought and drive and is likely caused "by a complex set of genetic and environmental factors," the clinic said. The statement also mentioned that Jackson underwent weight loss surgery in 2004 and said such a surgery can change how the body absorbs foods and medications, among other things.
The statement Monday was the most detailed to date about the congressman's mysterious medical leave, which began June 10. But it raised new questions about when the congressman can return to work.
A Jackson aide said last week that the congressman was expected back in the district within a matter of weeks, but Jackson's spokesmen declined to comment Monday.
His father, the Rev. Jesse Jackson, wouldn't say much about the diagnosis.
"I'm glad he's getting the treatment he needs and is responding well," the elder Jackson said, adding that "there's no timetable" for his recovery.
Experts and mental health advocates say many people are able to work and function in their daily lives while managing treatment.
Treatment includes medication and psychotherapy, according to the National Institute of Mental Health. The institute estimates about 5.7 million American adults suffer from the disorder, which can be a lifelong disease.
At least one other member of Congress has suffered from it while in office.
Former U.S. Rep. Patrick Kennedy of Rhode Island has talked openly about his lifelong struggles with bipolar disorder and addiction. He's was a leading voice in Congress for removing stigma linked with mental illness. The son of the late Massachusetts Sen. Edward Kennedy was a congressman for 16 years and retired last year.
The younger Kennedy was arrested in 2006 after an early morning car crash near the U.S. Capitol that he said he could not remember. After spending a month at Mayo for treatment of addiction and depression, Kennedy pleaded guilty to driving under the influence of prescription drugs.
"I had two of the biggest successes in politics after I went to treatment," Kennedy said, referring to getting nearly 70 percent of the re-election vote in 2006 and his legislative victory of getting a bill requiring mental health parity passed in 2008.
"It was because I ran toward the problem and not away from it. When I returned to my district, I spoke openly about it," he said.
Kennedy said he planned to visit Jackson on Thursday. He said he and Jackson had a lot in common: Both served on the House Appropriations Committee together and had famous fathers.
Jackson's office didn't announce his medical condition until about two weeks after he went on leave, and it initially described the problem as exhaustion. Later, his office disclosed that Jackson had "grappled with certain physical and emotional ailments privately for a long period of time." A statement from an unnamed doctor said Jackson had a "mood disorder."
Earlier this month, Jackson's office announced he was at Mayo and being treated for depression and gastrointestinal issues, after a transfer from the Sierra Tucson Treatment Center in Arizona.
Though the Mayo Clinic mentioned Jackson's weight loss surgery, its statement Monday stopped short of directly tying it to his mental health problems. Mayo Clinic spokeswoman Traci Klein declined to comment.
Dr. Jaime Ponce, president of the American Society for Metabolic and Bariatric Surgery, said there is no evidence that the type of surgery Jackson had can cause bipolar disorder. A deficiency of the nutrient thiamine can cause a brain condition that could mimic bipolar disorder, Ponce said, but "bipolar disorder is totally different."
Jackson underwent a duodenal switch procedure in 2004, which involves removing part of the stomach and rearranging the intestine so less food is absorbed. He lost 50 pounds.
Dr. Vivek Prachand, associate professor of surgery at University of Chicago, said people already taking medications for depression can undergo weight loss surgery but may need their medications adjusted afterward. Prachand added that surgery is a drastic change that can trigger an episode in someone with a history of depression.
Jackson aide Rick Bryant said last week that Jackson appeared in good spirits and wanted him to push forward on projects in the district, which includes Chicago neighborhoods and suburbs. Jackson, who first won office in 1995, is on the November ballot with two little-known candidates and is widely expected to win re-election.
The timing and manner in which the medical leave was handled has invited scrutiny.
Jackson is under a House Ethics Committee investigation for ties to imprisoned former Illinois Gov. Rod Blagojevich. Jackson's office announced his leave just days after a former fundraiser connected to the probe was arrested on federal medical fraud charges.
Jackson has denied wrongdoing.
Associated Press writers Carla K. Johnson in Chicago and Michelle R. Smith in Providence, R.I. contributed to this report.