The 93-year-old, who ruled the southern African nation for 37 years, resigned after the army and his ruling ZANU-PF party turned against him when it became clear that his 52-year-old wife, Grace, was being groomed as his successor.
Until recently, Mugabe had a reputation for extensive and expensive international travel, including regular medical trips to Singapore, a source of public anger among his impoverished citizens.
The official said he left Harare with Grace and aides on Monday evening, the official said.
He is expected to make a stop-over in Malaysia, where his daughter, Bona, is expecting a second child.
“He has gone for a routine medical trip to Singapore,” said the official, who has organized Mugabe’s security protection but who is not authorized to speak to the media.
“He was due for a check-up but events of the last few weeks made it impossible for him to travel.”
The trip means Mugabe will not be in Zimbabwe when ZANU-PF endorses President Emmerson Mnangagwa as its leader and presidential candidate for 2018 elections during a one-day special congress on Friday.
The security official would not say how Mugabe was traveling although the privately owned NewsDay newspaper said he was on a state-owned Air Zimbabwe plane.
Mugabe was granted immunity from prosecution and assured of his safety under his resignation deal, a source of frustration to many Zimbabweans who accused him of looting state coffers and destroying the economy during his time in power.
Another government official told Reuters in November that Mugabe had been due to travel to Singapore on Nov. 16 but was unable to leave because the military had confined him to his private home the previous day.
George Charamba, a senior information ministry official, declined to comment.
Under Zimbabwe’s Presidential Pension and Retirement Benefits Act, a former head of state is entitled to perks including limited foreign travel and medical insurance.
“These are very standard features of a retired president,” another government official said, trying to head off any controversy.
“You are making a storm out of nothing.”