Ko was most recently at Banco Santander, where she held the same position that she will take on at the U.S. investment bank.
She has previously worked for ANZ and Standard Chartered in Hong Kong, according to the announcement to staff that was also sent to Reuters.
Ko spent 15 years with Citigroup between 1989 and 2004, according to her LinkedIn profile.
In the announcement, Citigroup's Valentin Garger and Joy Cheng said the hiring should help the bank grow its market share in FIG transactions.
"Hong Kong financial institutions (group) is a significant and growing part of Corporate Banking in the country as well as a critical contributor to the broader Asia Pacific financial institutions franchise," it said.
Citigroup is aiming to have women account globally for at least 40% of their assistant vice president to managing director levels by the end of 2021.
In Asia, that level currently sits at 39%, according to a Citigroup spokesman.
Citigroup promoted 14 women, or 31% out of a total 45 managing directors named in Asia at the end of 2019, up from eight women the previous year, according to the spokesman.
(Reporting by Scott Murdoch)
By Scott Murdoch