In an internal memo sent to staff on Friday, the chief executive said the bank has been preparing for this outcome since last year.
In addition to the volatility and potential broader economic implications spurred by Brexit, it could also hurt global banks like Citi because many of them have large workforces in the UK.
Citi employs more than 9,000 people in the UK. Its shares are leading declines among the biggest US banks in the wake of the news - down 8% as of 12:23 p.m. in New York.
London-based employees who are not British citizens are now facing uncertainty about the future of their immigration status in the UK, and some firms might consider moving their European headquarters out of London.
Here is the full memo, which was cosigned by Jim Cowles, the CEO for Europe, Middle East and Africa:
"You are no doubt aware that yesterday, after a long and thorough debate, the British public has voted to leave the European Union.
"The UK is unlikely to formally exit the EU for at least two years, and during this period there will be no change in the way Citi is able to conduct its business.
"Last year, we created a group of senior leaders from across our businesses and functions to ensure we were prepared for this possible outcome. While the result of the vote is not what we would have preferred, our diligent work over the past six months means we can be confident that Citi is well positioned to continue to serve our clients.
"One thing that is very clear today is that our clients need our support. Yesterday's vote is having significant repercussions within the financial markets, which could affect the UK economy. This is a critical time for our client relationships, and we ask that you provide clients with all possible support as they work through how yesterday's vote affects them.
"For those EU nationals currently working in the UK and UK nationals in the EU, the outcome of the vote has no immediate effect on the terms of your employment. Should you have additional questions, please contact your manager or your local HR representative.
"We will update you regularly throughout this upcoming period. Thank you for your hard work, continued focus and all that you do for our clients."