Zardari's trip to Dubai last week has led to speculation - denied by the government - that he is losing his grip on power.
The associate said Zardari, 56, will stay under observation in the Gulf sheikdom for around two weeks before returning. He said there was no question that Zardari was too ill to return to office.
A "mini-stroke" is medically known as a transient ischemic attack.
It occurs when the blood supply to part of the brain is briefly interrupted, causing symptoms similar to a stroke but not as long-lasting.
The associate, who spoke on condition of anonymity because of the sensitive nature of the matter, said that Zardari's diagnosis had been made by the president's two physicians.
Earlier, officials had said that Zardari's trip was for routine medical tests. They then said he was having treatment related to a heart condition.
Questions about the president's future come as Pakistan is navigating a rough patch in relations with its most important ally, the United States, following NATO airstrikes last month that killed 24 Pakistani soldiers. The government called more than a dozen of its envoys back to Islamabad amid the crisis for a rare 2-day meeting that started Monday.
The attack has pushed the already strained U.S.-Pakistani relationship close to the breaking point, imperiling Washington's efforts to get Islamabad to cooperate on the Afghan war. Turmoil surrounding the Pakistani president would further complicate matters.