Spokeswoman Robin Urbanski said the airline did not know the cause of the problem, which affected the systems United uses to dispatch flights for departure.
The outage lasted from 8 a.m. to 10 a.m. CDT, when departures were able to resume after the computer functions returned.
United said about 268 domestic and international flights were delayed for an average of one and a half hours and 24 domestic flights were canceled. No international flights were canceled, Urbanski said.
The airline, a unit of Chicago-based UAL Corp., has about 3,600 flights daily.
"We are very sorry for the inconvenience today's computer outage caused our customers," Urbanski said Wednesday afternoon. "We continue to work hard to resume operations by [Thursday] morning and kindly ask for our customers' patience."
FAA spokesman Allen Kenitzer in Seattle confirmed that all United flights across its system had been grounded because of the computer problem.
At O'Hare International Airport, United's inbound and outbound flights were delayed between one and two hours, said spokeswoman Wendy Abrams.
No other airlines were affected.
Jocelyn Ashberg, from Capetown, South Africa, said she flew from Washington D.C. to Chicago Wednesday, only to sit on the tarmac at O'Hare for 1½ hours after the plane landed.
"We rushed to get the plane in Washington because we were late coming in because we were delayed in Johannesburg," Ashberg said. "Then when we got to Chicago, we just had to sit and sit and sit because nobody could tell us how long it was going to be ... we just had to sit like sardines in a can."
Michael McCarron, director of community affairs for the San Francisco International Airport, said about a dozen United flights were affected at the airport Wednesday morning before service resumed.
"Everything is back to normal right now," he said.
United is the airport's largest carrier.
The airline's hubs are O'Hare, Washington Dulles International and airports in Denver, Los Angeles and San Francisco.
By Ashley M. Heher