"My decision to step down was made in the last few months after much consideration," Levy told some United employees in an email that was seen by CNBC. "I have some interesting options, and now is the right time for me."
He said it was an "honor" to serve as CFO.
United CEO Oscar Munoz, who took the top position in September 2015 and was tested by repeated public relations missteps and sometimes skeptical investors, thanked Levy for his services to the carrier.
"He leaves the company in a stronger financial position and with a clear strategy and framework," Munoz said in a statement.
Munoz said that Laderman is highly qualified.
"Gerry's deep experience and financial acumen mean he is ideally suited to lead our financial organization during this transitional period," Munoz said. "Gerry will work with the rest of the team to pursue our financial strategy while we look for a permanent CFO."
United's stock, which rose 3 percent this year, has barely changed in the post-market trade.