Michael Pagan confirmed Cohen, 63, told his staff the 24-year-old woman is his daughter before talking to the news media, The Hill reported Friday.
"The reason why he didn't tell anyone is because he was trying to protect her and the family because they're private citizens," Pagan said.
Questions about the woman surfaced Wednesday after Cohen posted a message on Twitter to a user named Victoria Brink.
He tweeted "nice to know you were watchin SOTU (state of the union). Happy Valentines beautiful girl. Ilu."
The tweet was deleted within 15 minutes of being posted.
Cohen said he discovered his daughter's existence just three years ago, Politico reported.
"I googled her mother, found out she had a child and the math looked pretty accurate," Cohen said. "The mom told me we had a lot of catching up to do."