The cyberattackers penetrated the White House system for brief periods that allowed theft of information before U.S. government experts patched the system, the Financial Times was told.
Meanwhile, Newsweek reported the computer systems of Barack Obama and John McCain were hacked during the summer as they campaigned for the U.S. presidency, prompting an federal investigation.
The government official stressed that the White House hackers got into the unclassified computer network, not the more secure classified network.
Experts say they suspect the Chinese government sponsored the intrusion but had no proof.
The United States has increased efforts to improve computer system security, especially since Chinese hackers penetrated the Pentagon system last year.
Investigators told Obama's aides in August that McCain campaign's computer system was similarly compromised, which McCain aides confirmed, Newsweek reported.
After the warning from federal investigators, Obama campaign chief David Plouffe told Newsweek he took a call from White House Chief of Staff Josh Bolten, who said, "You have a real problem ... and you have to deal with it."
FBI and White House officials told the president-elect's campaign they believed a foreign entity or organization -- not an opponent -- wanted to collect information about both candidates' policy positions, Newsweek reported. Obama's tech experts later speculated the attacks may have originated in Russia or China.