The news was confirmed by a spokesman who also maintained the government's denial that Zimbabwe's long-serving ruler is suffering from prostate cancer.
George Charamba said Mugabe, Africa's oldest president, left Harare on Monday and would be back in the country for birthday celebrations on Saturday.
"This is a routine check-up, a routine cataract operation for his left eye whose date was set down more than a year ago and the president has gone out to fulfil that appointment," Charamba told Reuters on Tuesday.
"There is nothing more than that, nothing serious" he said, dismissing speculation that Mugabe is struggling with his health. "He had a right eye operation a couple of years ago and he is going to have the other attended to now."
A June 2008 US diplomatic cable released by WikiLeaks said Mugabe had prostate cancer that had spread to other organs.
He was apparently urged by his physician to step down in 2008 but has remained in the job.
Mugabe, who came to power when Zimbabwe gained independence from Britain in 1980, is one of Africa's longest serving leaders.
In an interview ahead of his 89th birthday last year Mugabe said he felt he had a "divine task" to lead. He went on to contest and win another five-year presidential term last July that will end in 2018 when he will be 94.
He denies charges by his opposition rivals that he rigged the election and that he has remained in power since 2000 through violence and cheating.
Critics say Mugabe, revered in his first decade in power as a liberation hero and a pragmatic leader, has squandered national goodwill by clinging to power and turning one of Africa's most promising economies into a basket case.
On Monday, the European Union kept sanctions on Mugabe and his wife, but suspended them on eight of Zimbabwe's most powerful military and political figures, in a cautious easing of policy after last year's disputed elections.
Mugabe has said his continued stay in power is aimed at advancing the aims of the liberation struggle for black economic empowerment, such as his policies of seizing white-owned commercial farms for blacks and forcing foreign-owned firms to surrender majority shares to locals.
In private, some in his ZANU-PF party have grudgingly accepted that Mugabe has manoeuvred himself into a position where he could end up president for life, a position that critics say he wants as security against possible prosecution for rights abuses.