The employee, who has not been named, had "an extensive history of visiting adult pornography websites".
Investigators found malware on many of the 9,000 pages he or she accessed.
The US Office of the Inspector General has recommended that the USGS blacklist "rogue" websites.
"Our analysis confirmed that many of the pornographic images were subsequently saved to an unauthorised USB device and personal Android cell phone," the report explained
As well as government computers, the employee's personal mobile was also found to be infected.